Certificate Course in Respiratory Medicine

Acute Respiratory Distress Syndrome (ARDS) is a form of non-cardiogenic pulmonary edema characterized by acute hypoxemic respiratory failure due to diffuse alveolar-capillary membrane injury, leading to increased pulmonary capillary permeability, fluid accumulation, and impaired gas exchange. It is a critical care emergency and high-yield topic on USMLE Step 2 CK, often associated with sepsis, pneumonia, aspiration, trauma, pancreatitis, or massive transfusion. Patients with ARDS present with severe dyspnea, tachypnea, hypoxemia, and bilateral pulmonary infiltrates on chest imaging, often progressing to respiratory failure requiring mechanical ventilation.

The diagnosis is clinical and supported by the Berlin criteria, which require: (1) acute onset within 1 week of a known insult; (2) bilateral opacities on chest X-ray or CT not fully explained by effusion, lobar collapse, or nodules; (3) respiratory failure not due to cardiac causes (i.e., no evidence of volume overload or elevated left atrial pressure); and (4) PaO₂/FiO₂ ratio ≤300 mmHg, with severity classified as mild (200–300), moderate (100–200), or severe (<100).

Management is supportive and centers on lung-protective mechanical ventilation, with low tidal volumes (6 mL/kg of predicted body weight), permissive hypercapnia, and maintaining plateau pressures <30 cm H₂O to reduce ventilator-induced lung injury. Positive end-expiratory pressure (PEEP) is applied to prevent alveolar collapse, and in refractory cases, prone positioning or neuromuscular blockade may improve oxygenation. Fluid management is conservative once hemodynamic stability is achieved to minimize further pulmonary edema. Treating the underlying cause (e.g., antibiotics for sepsis or aspiration) is essential.

Aspergillosis refers to a spectrum of diseases caused by the Aspergillus species, a ubiquitous mold found in the environment, and it becomes clinically significant primarily in immunocompromised hosts or those with pre-existing lung disease. It is a high-yield topic for USMLE Step 2 CK, especially in vignettes involving neutropenic patients, transplant recipients, or individuals with chronic lung conditions like tuberculosis or COPD. The clinical manifestations are broadly categorized into allergic bronchopulmonary aspergillosis (ABPA), aspergilloma (fungus ball), and invasive aspergillosis.

ABPA is a hypersensitivity reaction seen mostly in patients with asthma or cystic fibrosis, presenting with worsening wheezing, cough with brownish sputum, fever, and transient pulmonary infiltrates. Laboratory findings include eosinophilia, elevated total and Aspergillus-specific IgE, and central bronchiectasis on imaging. Treatment includes oral corticosteroids and sometimes itraconazole for antifungal suppression.

Aspergilloma occurs when Aspergillus colonizes a pre-existing lung cavity, typically due to prior TB or sarcoidosis. It often presents with hemoptysis, which can be life-threatening. Chest imaging reveals a mobile, rounded mass within a cavity, often with an air crescent. Management is generally surgical resection if symptomatic; antifungals are not always effective for simple aspergilloma.

Invasive aspergillosis is the most severe form, seen in patients with profound immunosuppression, especially those with prolonged neutropenia, hematologic malignancies, or post-transplantation. It presents with fever unresponsive to antibiotics, pleuritic chest pain, hemoptysis, and dyspnea. CT chest typically shows nodules with surrounding ground-glass opacities ("halo sign"), and diagnosis is supported by positive galactomannan antigen testing, fungal cultures, or biopsy. Treatment is urgent and involves voriconazole as the first-line agent, sometimes combined with echinocandins in severe cases.

Bronchiolitis refers to inflammation of the small airways (bronchioles) and is most commonly associated with acute viral infection in infants, but in adults, it encompasses a broader category of chronic and subacute inflammatory airway diseases involving the terminal and respiratory bronchioles. On USMLE Step 2 CK, bronchiolitis is tested both in pediatric settings (acute viral bronchiolitis) and adult internal medicine, where it often appears in the context of bronchiolitis obliterans, respiratory bronchiolitis, or bronchiolitis associated with transplantation or inhalational injury.

In infants and young children, acute bronchiolitis is typically caused by respiratory syncytial virus (RSV), peaking in the winter months. It presents with nasal congestion, rhinorrhea, cough, wheezing, tachypnea, and increased work of breathing, and is the most common cause of hospitalization in children under 2 years. Diagnosis is clinical, and chest X-ray may show hyperinflation and patchy atelectasis, but imaging is usually unnecessary. Treatment is supportive, including nasal suctioning, hydration, and oxygen if needed. Bronchodilators, corticosteroids, and antibiotics are not routinely recommended. Palivizumab, a monoclonal antibody, is used for RSV prophylaxis in high-risk infants (e.g., preterm infants, congenital heart disease).

In adults, bronchiolitis obliterans (also known as constrictive bronchiolitis) is a rare but serious form of chronic obstructive lung disease caused by fibrotic narrowing of the small airways. It can occur after lung or bone marrow transplantation, rheumatologic diseases (e.g., rheumatoid arthritis), or toxic inhalational exposure (e.g., diacetyl in microwave popcorn factory workers). Symptoms include progressive dyspnea, dry cough, and obstructive pattern on spirometry (low FEV₁ with normal FVC). HRCT scan shows mosaic attenuation, air trapping, and bronchial wall thickening. Lung biopsy may be required for definitive diagnosis. Treatment is often challenging and may involve immunosuppression (e.g., corticosteroids), macrolides, and lung transplantation in severe cases.

Bronchiectasis is a chronic structural lung disease characterized by permanent dilation and destruction of the bronchial walls, leading to impaired mucociliary clearance, chronic productive cough, and recurrent respiratory infections. It is a high-yield topic for USMLE Step 2 CK, particularly in patients with chronic cough, copious sputum production, or recurrent episodes of bronchitis or pneumonia. The condition may be focal, due to obstruction or localized infection, or diffuse, associated with systemic diseases like cystic fibrosis (CF), primary ciliary dyskinesia (e.g., Kartagener syndrome), immunodeficiency (e.g., IgA deficiency, hypogammaglobulinemia), allergic bronchopulmonary aspergillosis (ABPA), or chronic inflammatory lung diseases.

Patients typically present with a daily productive cough, purulent sputum, dyspnea, hemoptysis, and recurrent pulmonary infections. On auscultation, coarse crackles (rales) and rhonchi may be heard. Physical exam may show digital clubbing, especially in advanced disease. Acute exacerbations often resemble bacterial pneumonia and are triggered by organisms such as Pseudomonas aeruginosa, Haemophilus influenzae, or Streptococcus pneumoniae.

Diagnosis is confirmed with high-resolution CT (HRCT) of the chest, which shows tram-track markings, signet ring sign, and cystic bronchial dilation. Pulmonary function tests (PFTs) usually reveal an obstructive pattern with reduced FEV₁/FVC ratio. Additional testing should include sputum cultures, immunoglobulin levels, testing for CF (sweat chloride or CFTR genotyping), and workup for ABPA (e.g., elevated IgE, eosinophilia, positive Aspergillus-specific IgG/IgE).

Management focuses on controlling infection, reducing inflammation, and promoting airway clearance. This includes airway clearance techniques (chest physiotherapy, postural drainage), bronchodilators, inhaled corticosteroids, and long-term antibiotics (e.g., azithromycin) in patients with frequent exacerbations. Acute infections are treated with culture-guided antibiotics, and inhaled antibiotics may be used in patients with chronic Pseudomonas colonization. Surgical resection is rarely indicated and reserved for localized, severe, and refractory cases.

Bronchogenic carcinoma, broadly referring to primary lung cancer, arises from the epithelial lining of the bronchi and is a major cause of cancer-related mortality worldwide. It is a high-yield malignancy for USMLE Step 2 CK, often tested in the context of smoking history, paraneoplastic syndromes, and radiographic findings. The two main categories are non–small cell lung carcinoma (NSCLC), which accounts for approximately 85% of cases, and small cell lung carcinoma (SCLC), which is more aggressive and often presents with early metastasis and paraneoplastic features.

NSCLC includes adenocarcinoma (most common overall and in non-smokers, typically peripheral), squamous cell carcinoma (strongly associated with smoking, typically central), and large cell carcinoma. SCLC, a neuroendocrine tumor, almost exclusively affects smokers and is typically centrally located, rapidly growing, and often unresectable at diagnosis. It frequently causes paraneoplastic syndromes, including SIADH (hyponatremia), ectopic ACTH production (Cushing syndrome), and Lambert-Eaton myasthenic syndrome.

Patients may present with chronic cough, hemoptysis, chest pain, weight loss, or dyspnea. Some are asymptomatic and found incidentally on imaging. Horner’s syndrome (ptosis, miosis, anhidrosis) may suggest a Pancoast tumor at the lung apex, often associated with shoulder pain and brachial plexus involvement. Superior vena cava (SVC) syndrome presents with facial swelling, distended neck veins, and dyspnea, especially in SCLC.

Diagnosis begins with chest X-ray followed by CT scan of the chest, and confirmed with bronchoscopy with biopsy, CT-guided biopsy, or sputum cytology, depending on tumor location. PET scan and brain MRI are used for staging, especially in SCLC, which is usually classified as limited or extensive stage, while NSCLC follows TNM staging.

Management depends on histology and stage. Surgical resection is preferred for early-stage NSCLC (especially stage I or II), often combined with adjuvant chemotherapy or radiation. SCLC is usually treated with chemotherapy and radiation due to early dissemination. Targeted therapies and immunotherapy (e.g., EGFR inhibitors, PD-1/PD-L1 inhibitors) play a growing role in NSCLC based on molecular markers.

Cystic fibrosis (CF) is a multisystem, autosomal recessive disorder caused by mutations in the CFTR gene on chromosome 7, leading to defective chloride transport across epithelial cells. This results in thick, viscous secretions affecting the lungs, pancreas, gastrointestinal tract, hepatobiliary system, and reproductive organs. It is one of the most important genetic diseases tested on USMLE Step 2 CK, particularly in young patients with chronic respiratory symptoms, malabsorption, and failure to thrive. The most common mutation is ΔF508, which causes misfolding and degradation of the CFTR protein.

Pulmonary involvement is the major cause of morbidity and mortality. Patients develop recurrent respiratory infections, chronic productive cough, nasal polyps, and bronchiectasis. Pseudomonas aeruginosa is a hallmark pathogen in chronic colonization and recurrent infections. Progressive lung disease leads to hypoxia, digital clubbing, and ultimately respiratory failure. Gastrointestinal manifestations include pancreatic insufficiency, resulting in steatorrhea, fat-soluble vitamin deficiencies (A, D, E, K), and malnutrition. CF-related diabetes mellitus and distal intestinal obstruction syndrome (DIOS) are common in adolescents and adults. In the hepatobiliary system, biliary cirrhosis and gallstones may occur. Nearly all males are infertile due to congenital absence of the vas deferens.

Diagnosis is confirmed by a sweat chloride test showing elevated chloride concentration (>60 mmol/L), or by genetic testing for CFTR mutations. Newborn screening programs help identify cases early. Pulmonary function tests show obstructive lung disease with decreased FEV₁ and FEV₁/FVC ratio.

Management is multidisciplinary and includes airway clearance techniques (e.g., chest physiotherapy, oscillatory vests), bronchodilators, mucolytics (like dornase alfa), and inhaled antibiotics (e.g., tobramycin for Pseudomonas). Pancreatic enzyme replacement, nutritional support, and fat-soluble vitamin supplementation are crucial for GI management. Newer CFTR modulators like ivacaftor, lumacaftor, and elexacaftor/tezacaftor/ivacaftor have dramatically improved outcomes in patients with specific mutations. Lung transplantation may be considered in end-stage disease.

Mesothelioma is a rare but aggressive malignancy of the mesothelial cells, most commonly arising from the pleura, and is strongly associated with asbestos exposure, making it a classic occupational lung disease tested on USMLE Step 2 CK. Unlike bronchogenic carcinoma, which is also linked to asbestos but has a stronger association with smoking, mesothelioma is not significantly increased by smoking alone. The disease typically has a long latency period (20–40 years) after asbestos exposure and most often presents in older men with occupational histories involving shipbuilding, insulation, construction, or asbestos mining.

Patients commonly present with progressive dyspnea, non-pleuritic chest pain, and pleural effusion, often recurrent and exudative, which may be hemorrhagic. Other findings include weight loss, fatigue, and digital clubbing. On physical exam, decreased breath sounds and dullness to percussion over the effusion are common. Imaging studies such as chest X-ray may show a unilateral pleural effusion or pleural thickening, while CT scan reveals nodular pleural masses, pleural rind, or encasement of the lung, particularly along the diaphragm or mediastinal pleura.

Thoracentesis may yield hemorrhagic fluid, but cytology is often non-diagnostic, necessitating pleural biopsy, preferably via video-assisted thoracoscopic surgery (VATS), for histopathologic confirmation. On biopsy, mesothelioma demonstrates atypical mesothelial cells, and immunohistochemical staining is used to distinguish it from metastatic adenocarcinoma. The tumor can be epithelioid (most common and better prognosis), sarcomatoid, or biphasic.

Management is challenging due to late-stage presentation. Treatment options include chemotherapy (e.g., cisplatin with pemetrexed), radiation, and surgical resection (pleurectomy/decortication or extrapleural pneumonectomy) in selected cases. Prognosis is poor, with median survival often less than 12 months in advanced cases.

Tropical Pulmonary Eosinophilia (TPE) is a hypersensitivity reaction to filarial parasites, most commonly Wuchereria bancrofti or Brugia malayi, and is seen predominantly in individuals living in or returning from endemic tropical and subtropical regions such as India, Southeast Asia, Africa, and South America. It is a high-yield pulmonary eosinophilic disorder for USMLE Step 2 CK, particularly in vignettes involving chronic respiratory symptoms, eosinophilia, and travel or residency in endemic areas. TPE represents an immune hyperresponsiveness to microfilariae trapped in the lungs and is distinct from active filarial infection causing lymphedema or elephantiasis.

Patients typically present with chronic dry cough, wheezing, dyspnea, low-grade fever, weight loss, and fatigue, often mimicking asthma or tuberculosis. Nocturnal symptoms are common, reflecting the nocturnal periodicity of the microfilariae. On physical examination, bilateral wheezing or crackles may be heard, but findings can be nonspecific.

Laboratory studies reveal marked peripheral eosinophilia (often >3,000 cells/μL), elevated total IgE, and positive anti-filarial antibodies. Chest imaging may show reticulonodular infiltrates, miliary mottling, or interstitial lung patterns, especially in the mid and lower zones. Pulmonary function tests often demonstrate a restrictive pattern or mixed ventilatory defect. Microfilariae are typically absent in the blood, as they are rapidly cleared by the immune system, making serology the key diagnostic tool.

Treatment is with diethylcarbamazine (DEC), which is both diagnostic and therapeutic, typically given for 3 weeks. Patients often demonstrate a dramatic clinical and hematologic response, including resolution of cough and normalization of eosinophil counts. In corticosteroid-refractory or relapsing cases, adjunctive corticosteroids may be used to reduce inflammation.

Pleural effusion refers to the abnormal accumulation of fluid in the pleural space, and is a high-yield clinical finding on USMLE Step 2 CK, often associated with dyspnea, pleuritic chest pain, and dullness to percussion on physical exam. It can be classified as either a transudative or exudative effusion, based on the underlying mechanism and fluid characteristics, which guides diagnosis and management. Transudative effusions are caused by systemic factors that alter hydrostatic or oncotic pressure, such as congestive heart failure (most common cause overall), cirrhosis (hepatic hydrothorax), and nephrotic syndrome. Exudative effusions, in contrast, result from inflammation or increased vascular permeability, as seen in infection (e.g., pneumonia causing parapneumonic effusion or empyema), malignancy, pulmonary embolism, tuberculosis, or autoimmune diseases (e.g., lupus, rheumatoid arthritis).

Patients typically present with progressive shortness of breath, pleuritic chest pain, and cough. On exam, findings include decreased breath sounds, dullness to percussion, decreased tactile fremitus, and sometimes pleural friction rub. Chest X-ray shows blunting of the costophrenic angle and meniscus sign, and ultrasound is often used to guide thoracentesis. CT chest may be helpful in complicated cases or to assess underlying lung pathology.

Thoracentesis is the key diagnostic step, especially for new or unexplained effusions. The fluid should be analyzed for protein, LDH, glucose, cell count with differential, Gram stain and culture, cytology, and pH. Light’s criteria are used to distinguish exudate from transudate: the fluid is exudative if one or more of the following are true:

• Pleural fluid protein/serum protein ratio > 0.5

• Pleural fluid LDH/serum LDH ratio > 0.6

• Pleural fluid LDH > 2/3 of the upper limit of normal for serum LDH

Management depends on the etiology. Transudative effusions are treated by managing the underlying condition (e.g., diuretics for heart failure), and thoracentesis is usually not needed unless symptomatic. Exudative effusions may require antibiotics, chest tube drainage (for empyema), pleurodesis, or oncologic therapies. Recurrent malignant effusions may need indwelling pleural catheters.

Lung abscess is a localized, necrotizing infection of the lung parenchyma that leads to formation of a cavity filled with pus, typically due to aspiration of oropharyngeal contents into the lower airways. It is a high-yield pulmonary infection on USMLE Step 2 CK, especially in patients with poor dental hygiene, alcoholism, seizure disorders, or altered consciousness, all of which predispose to aspiration. The posterior segments of the upper lobes and superior segments of the lower lobes are most commonly affected due to gravitational drainage during recumbency.

Patients usually present with insidious onset of fever, productive cough, night sweats, weight loss, and foul-smelling purulent sputum—a hallmark clue pointing to anaerobic infection. Physical exam may reveal crackles, dullness to percussion, or signs of consolidation. In some cases, hemoptysis or pleuritic chest pain may also occur. The most common pathogens are anaerobic bacteria (e.g., Peptostreptococcus, Fusobacterium, Bacteroides), but aerobic organisms like Staphylococcus aureus (including MRSA), Klebsiella, and Pseudomonas may be involved, especially in hospitalized or immunocompromised patients.

Chest X-ray typically shows a thick-walled cavitary lesion with air-fluid level, but CT chest is more sensitive and helps differentiate it from malignancy, tuberculosis, or empyema. Sputum cultures may guide therapy, though anaerobes are often hard to isolate. Bronchoscopy may be considered in non-resolving cases or to exclude obstructing lesions.

Management involves prolonged antibiotic therapy, typically for 4–6 weeks. First-line treatment includes clindamycin or ampicillin-sulbactam, both of which provide anaerobic coverage. In patients with risk of resistant organisms or hospital-acquired infection, broader-spectrum antibiotics (e.g., piperacillin-tazobactam or carbapenems) may be used. Surgical drainage or lobectomy is rarely required but may be indicated for refractory cases or complications like massive hemoptysis or rupture into the pleural space.

Chronic Obstructive Pulmonary Disease (COPD) is a progressive, irreversible airflow limitation disorder characterized by chronic bronchitis and/or emphysema, most commonly caused by long-term smoking, and is a core pulmonary topic on USMLE Step 2 CK. It presents with dyspnea, chronic cough, sputum production, and progressive exercise intolerance, especially in middle-aged to older adults with a history of smoking or exposure to biomass fuels. Chronic bronchitis is defined clinically as a productive cough lasting at least 3 months for 2 consecutive years, while emphysema is characterized by destruction of alveolar walls and enlarged airspaces, leading to decreased elastic recoil and air trapping.

On physical examination, findings may include barrel-shaped chest, pursed-lip breathing, accessory muscle use, decreased breath sounds, and prolonged expiratory phase. Severe disease may show signs of cor pulmonale (right heart failure) or cyanosis. Pulmonary function tests (PFTs) are diagnostic, showing a reduced FEV₁/FVC ratio (<70%), indicating airflow obstruction. Post-bronchodilator testing confirms irreversibility. In emphysema-predominant disease, DLCO may be reduced, while in chronic bronchitis, DLCO is often preserved. Chest X-ray may show hyperinflated lungs, flattened diaphragm, and increased retrosternal airspace.

Management depends on symptom severity and exacerbation history, based on GOLD (Global Initiative for Chronic Obstructive Lung Disease) guidelines. All patients benefit from smoking cessation, vaccinations (influenza, pneumococcal, COVID-19), and pulmonary rehabilitation. Short-acting bronchodilators (SABA/SAMA) are used for symptom relief. For persistent symptoms, long-acting bronchodilators (LABA or LAMA) are added. Inhaled corticosteroids (ICS) are reserved for patients with frequent exacerbations or eosinophilic phenotype. Oxygen therapy improves survival in patients with chronic hypoxemia (PaO₂ ≤ 55 mmHg or SaO₂ ≤ 88%). In select cases, surgical options such as lung volume reduction surgery or lung transplantation may be considered.

Exacerbations are acute worsening of symptoms, often triggered by respiratory infections. Management includes bronchodilators, systemic corticosteroids, and antibiotics if sputum is purulent or signs of infection are present. Hospitalization is required for severe cases or respiratory failure, and non-invasive ventilation (e.g., BiPAP) is used to prevent intubation.

Chronic Obstructive Pulmonary Disease (COPD) is a progressive, irreversible airflow limitation disorder characterized by chronic bronchitis and/or emphysema, most commonly caused by long-term smoking, and is a core pulmonary topic on USMLE Step 2 CK. It presents with dyspnea, chronic cough, sputum production, and progressive exercise intolerance, especially in middle-aged to older adults with a history of smoking or exposure to biomass fuels. Chronic bronchitis is defined clinically as a productive cough lasting at least 3 months for 2 consecutive years, while emphysema is characterized by destruction of alveolar walls and enlarged airspaces, leading to decreased elastic recoil and air trapping.

On physical examination, findings may include barrel-shaped chest, pursed-lip breathing, accessory muscle use, decreased breath sounds, and prolonged expiratory phase. Severe disease may show signs of cor pulmonale (right heart failure) or cyanosis. Pulmonary function tests (PFTs) are diagnostic, showing a reduced FEV₁/FVC ratio (<70%), indicating airflow obstruction. Post-bronchodilator testing confirms irreversibility. In emphysema-predominant disease, DLCO may be reduced, while in chronic bronchitis, DLCO is often preserved. Chest X-ray may show hyperinflated lungs, flattened diaphragm, and increased retrosternal airspace.

Management depends on symptom severity and exacerbation history, based on GOLD (Global Initiative for Chronic Obstructive Lung Disease) guidelines. All patients benefit from smoking cessation, vaccinations (influenza, pneumococcal, COVID-19), and pulmonary rehabilitation. Short-acting bronchodilators (SABA/SAMA) are used for symptom relief. For persistent symptoms, long-acting bronchodilators (LABA or LAMA) are added. Inhaled corticosteroids (ICS) are reserved for patients with frequent exacerbations or eosinophilic phenotype. Oxygen therapy improves survival in patients with chronic hypoxemia (PaO₂ ≤ 55 mmHg or SaO₂ ≤ 88%). In select cases, surgical options such as lung volume reduction surgery or lung transplantation may be considered.

Exacerbations are acute worsening of symptoms, often triggered by respiratory infections. Management includes bronchodilators, systemic corticosteroids, and antibiotics if sputum is purulent or signs of infection are present. Hospitalization is required for severe cases or respiratory failure, and non-invasive ventilation (e.g., BiPAP) is used to prevent intubation.

Pulmonary metastasis refers to the spread of malignant cells to the lungs from a primary tumor located elsewhere in the body, and it is a common site of metastatic disease due to the lungs’ rich vascular supply and filtering function of the pulmonary capillary bed. It is a high-yield topic on USMLE Step 2 CK, particularly in the evaluation of patients with known malignancies who develop new respiratory symptoms or abnormal chest imaging. The most common primary tumors that metastasize to the lungs include breast, colon, renal cell carcinoma, melanoma, sarcomas, and head and neck cancers.

Pulmonary metastases may be asymptomatic and discovered incidentally on imaging, or they may present with cough, hemoptysis, dyspnea, pleuritic chest pain, or signs of malignant pleural effusion. Systemic symptoms such as weight loss and fatigue may also be present in advanced disease. On chest X-ray, metastases typically appear as multiple, bilateral, well-circumscribed “cannonball” nodules, though they may also manifest as lymphangitic spread, solitary nodules, or endobronchial lesions. CT chest is more sensitive and provides detailed characterization.

Diagnosis involves correlating imaging findings with history of known malignancy, and tissue confirmation is often done via CT-guided biopsy, bronchoscopy with biopsy, or video-assisted thoracoscopic surgery (VATS) if needed. PET scan may help evaluate for systemic disease burden. In cases with pleural involvement, thoracentesis with cytology can help establish diagnosis.

Management depends on the type of primary cancer, extent of metastasis, and overall prognosis. Treatment options may include systemic chemotherapy, targeted therapy, immunotherapy, or in select cases, surgical resection (e.g., in oligometastatic disease). Palliative care and symptom management become central in widespread or inoperable cases. Malignant pleural effusions may require pleurodesis or indwelling pleural catheters to relieve symptoms.

Pulmonary Alveolar Proteinosis (PAP) is a rare lung disease characterized by the accumulation of lipoproteinaceous material within the alveoli, which impairs gas exchange and results in progressive respiratory symptoms. It is a high-yield yet uncommon pulmonary condition on USMLE Step 2 CK, particularly in patients with insidious onset of dyspnea, cough, and diffuse alveolar infiltrates on imaging. PAP can be classified into three main types: autoimmune (primary), which is the most common form and caused by anti–GM-CSF antibodies; secondary, associated with hematologic malignancies, inhalational exposures, or infections; and congenital, due to surfactant protein mutations, typically seen in neonates.

Clinically, patients with PAP present with progressive exertional dyspnea, nonproductive cough, and sometimes fatigue, weight loss, or low-grade fever. Physical examination may be unremarkable or reveal crackles and signs of hypoxemia in advanced cases. Notably, the disease may mimic pneumonia or interstitial lung disease, often delaying diagnosis.

Chest imaging is central to diagnosis. Chest X-ray may show bilateral, symmetric alveolar infiltrates, while high-resolution CT (HRCT) typically reveals the classic “crazy paving” pattern, which consists of ground-glass opacities with interlobular septal thickening. To confirm the diagnosis, bronchoalveolar lavage (BAL) yields a milky fluid containing PAS-positive lipoproteinaceous material. Lung biopsy is rarely needed but may be performed if diagnosis is uncertain.

Management depends on the severity and underlying cause. In autoimmune PAP, the standard treatment is whole lung lavage, where saline is used to physically wash out the alveolar contents. Recombinant GM-CSF therapy (inhaled or subcutaneous) is an emerging treatment targeting the underlying pathophysiology. In secondary PAP, addressing the underlying condition (e.g., discontinuing toxic exposures or treating malignancy) is essential. Infections, particularly opportunistic ones, must be monitored and treated promptly.

Pneumonia is an acute infection of the lung parenchyma leading to inflammation and consolidation of the alveoli, and is one of the most commonly encountered respiratory conditions in clinical medicine and on USMLE Step 2 CK. It presents with fever, productive cough, pleuritic chest pain, dyspnea, and crackles or bronchial breath sounds on lung auscultation. Pneumonia is typically classified into community-acquired (CAP), hospital-acquired (HAP), and ventilator-associated pneumonia (VAP) based on the setting and timing of onset, which also guides empiric antibiotic therapy.

Streptococcus pneumoniae remains the most common cause of CAP, presenting with sudden-onset fever, cough with rust-colored sputum, and lobar consolidation on chest X-ray. Other important pathogens include Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumoniae (atypical pneumonia with dry cough and interstitial infiltrates), Legionella (associated with hyponatremia, GI symptoms, and contaminated water sources), and viral causes like influenza or COVID-19. In HAP and VAP, gram-negative bacilli (e.g., Pseudomonas aeruginosa, Klebsiella, E. coli) and MRSA are more common.

Diagnosis is clinical but supported by chest X-ray, which may show lobar consolidation, interstitial infiltrates, or cavitary lesions, depending on the pathogen. Sputum Gram stain and culture, blood cultures, and urinary antigen tests (for Legionella and S. pneumoniae) can help identify the causative agent. Pulse oximetry or ABG is used to assess oxygenation.

Management is based on severity. Outpatients with CAP are treated empirically with amoxicillin + doxycycline or a macrolide (e.g., azithromycin). Inpatients without ICU admission may receive ceftriaxone plus azithromycin or a respiratory fluoroquinolone (e.g., levofloxacin). ICU patients or those at risk for resistant organisms require broader coverage, often including vancomycin or piperacillin-tazobactam. Supportive care, including oxygen, fluids, and antipyretics, is also essential.

Complications include parapneumonic effusion, empyema, lung abscess, sepsis, and acute respiratory distress syndrome (ARDS). Vaccination with pneumococcal vaccines (PCV20, PPSV23) and annual influenza vaccination is critical for prevention, especially in the elderly, immunocompromised, or those with chronic comorbidities.

Pulmonary hypertension (PH) is defined as mean pulmonary arterial pressure ≥25 mmHg at rest, measured by right heart catheterization, and represents a progressive condition characterized by increased pulmonary vascular resistance leading to right ventricular hypertrophy, dilation, and eventual right heart failure (cor pulmonale). It is a high-yield cardiopulmonary disorder for USMLE Step 2 CK, often tested through presentations involving exertional dyspnea, fatigue, chest pain, syncope, or signs of right-sided heart failure such as peripheral edema, hepatomegaly, and jugular venous distension.

Pulmonary hypertension is classified into five WHO groups based on etiology:

• Group 1: Pulmonary arterial hypertension (PAH) — idiopathic, heritable, or secondary to conditions like connective tissue diseases (e.g., systemic sclerosis), HIV, portal hypertension, or drug toxicity (e.g., fenfluramine).

• Group 2: Due to left heart disease, such as heart failure with preserved or reduced ejection fraction or mitral valve disease.

• Group 3: Due to chronic lung diseases like COPD, interstitial lung disease, or sleep apnea.

• Group 4: Due to chronic thromboembolic pulmonary hypertension (CTEPH).

• Group 5: Miscellaneous causes including sarcoidosis, hematologic disorders, and metabolic conditions.

Initial evaluation includes echocardiography, which estimates pulmonary artery pressures and assesses right heart structure. Right heart catheterization is required for definitive diagnosis and classification. ECG may show signs of right heart strain (e.g., right axis deviation, RV hypertrophy), and chest X-ray may reveal enlarged pulmonary arteries or right atrial enlargement. V/Q scanning is preferred for screening CTEPH, and PFTs and polysomnography help evaluate lung disease or sleep apnea.

Management depends on the underlying cause. In Group 1 (PAH), treatment includes vasodilator therapy with endothelin receptor antagonists (e.g., bosentan), phosphodiesterase-5 inhibitors (e.g., sildenafil), prostacyclin analogs (e.g., epoprostenol), and calcium channel blockers in vasoreactive patients. Groups 2 and 3 are treated by managing the underlying cardiac or pulmonary disease, while anticoagulation and surgical thromboendarterectomy are used in Group 4 CTEPH. Supplemental oxygen, diuretics, exercise, and salt restriction are often part of supportive care across all groups.

Pulmonary function tests (PFTs) are a fundamental set of diagnostic tools used to evaluate lung volumes, capacities, airflow, and gas exchange, and they play a crucial role in the diagnosis and differentiation of obstructive and restrictive lung diseases. Mastery of interpreting PFTs is essential for USMLE Step 2 CK, particularly in clinical vignettes involving patients with dyspnea, chronic cough, or abnormal chest imaging. The most commonly assessed parameters include FEV₁ (forced expiratory volume in 1 second), FVC (forced vital capacity), and the FEV₁/FVC ratio, which are obtained via spirometry.

In obstructive lung diseases such as COPD and asthma, there is decreased airflow due to airway narrowing, leading to a reduced FEV₁/FVC ratio (<70%), with a more pronounced drop in FEV₁. In asthma, the airflow limitation is reversible, demonstrated by a >12% and >200 mL increase in FEV₁ after bronchodilator therapy, whereas in COPD, the obstruction is non-reversible. Additional findings in obstructive disease may include increased total lung capacity (TLC) and residual volume (RV) due to air trapping, and reduced DLCO in emphysema.

In restrictive lung diseases such as interstitial lung disease, pulmonary fibrosis, or neuromuscular disorders, the lungs are unable to expand fully, resulting in reduced TLC, FVC, and FEV₁, but the FEV₁/FVC ratio is normal or increased. DLCO is typically decreased in parenchymal lung disease but normal in extrapulmonary restriction like obesity or myasthenia gravis.

DLCO (diffusing capacity of the lungs for carbon monoxide) is a valuable measurement of the lung’s ability to exchange gases. It is reduced in emphysema, interstitial lung disease, pulmonary hypertension, and anemia, while it may be increased in asthma, pulmonary hemorrhage, or polycythemia.

PFTs are also used for preoperative risk assessment, monitoring response to therapy, and evaluating occupational lung disease or drug toxicity (e.g., from amiodarone or bleomycin). Interpretation must consider spirometry patterns, lung volumes, and DLCO collectively to avoid misdiagnosis.

Respiratory failure is a life-threatening condition in which the respiratory system fails to maintain adequate gas exchange, resulting in hypoxemia (PaO₂ < 60 mmHg), hypercapnia (PaCO₂ > 50 mmHg), or both. It is a high-yield emergency topic for USMLE Step 2 CK, often encountered in clinical vignettes involving acute dyspnea, altered mental status, or hypoxia in settings such as COPD exacerbations, ARDS, pneumonia, pulmonary embolism, neuromuscular disorders, or drug overdoses. Respiratory failure is broadly classified into Type I (hypoxemic) and Type II (hypercapnic) failure, with distinct mechanisms and causes.

Type I respiratory failure is characterized by low PaO₂ with normal or low PaCO₂, due to ventilation-perfusion (V/Q) mismatch, shunt, or diffusion impairment, commonly seen in ARDS, pneumonia, pulmonary edema, and pulmonary embolism. Type II respiratory failure, on the other hand, results from alveolar hypoventilation, leading to both hypoxemia and hypercapnia, and occurs in COPD exacerbations, neuromuscular diseases (e.g., myasthenia gravis, Guillain-Barré), central respiratory depression (e.g., opioids, CNS trauma), or severe obesity (obesity hypoventilation syndrome).

Clinically, patients may present with dyspnea, tachypnea, cyanosis, confusion, lethargy, or even respiratory arrest. Physical exam may reveal use of accessory muscles, decreased breath sounds, or altered mental status due to CO₂ retention. Arterial blood gas (ABG) is the key diagnostic test, allowing identification of the type of respiratory failure and guiding therapy. Chest X-ray, ECG, pulse oximetry, and labs help evaluate underlying causes and rule out complications.

Management depends on the underlying cause and severity. In Type I failure, treatment focuses on oxygen therapy and treating the primary disease (e.g., antibiotics for pneumonia, diuretics for pulmonary edema). In Type II failure, care must be taken with oxygen administration to avoid suppressing respiratory drive, especially in COPD; instead, non-invasive positive pressure ventilation (e.g., BiPAP) is often the first-line therapy. In cases of worsening acidosis or mental status changes, endotracheal intubation and mechanical ventilation may be required. Treating the underlying etiology—whether it is infection, airway obstruction, muscle weakness, or central depression—is essential for resolution.

Pneumothorax is the presence of air in the pleural space, leading to partial or complete collapse of the lung, and is a high-yield pulmonary emergency on USMLE Step 2 CK, often tested through sudden-onset unilateral pleuritic chest pain and dyspnea, particularly in young, thin males or patients with underlying lung disease. Pneumothorax is classified into primary spontaneous (occurs without underlying lung pathology), secondary spontaneous (in patients with lung disease like COPD), traumatic (due to blunt or penetrating injury), and tension pneumothorax, which is a life-threatening emergency. In primary spontaneous pneumothorax, rupture of subpleural blebs causes air to leak into the pleural space, commonly affecting tall, thin males in their 20s. Secondary spontaneous pneumothorax occurs in patients with structural lung diseases, such as emphysema, cystic fibrosis, or tuberculosis.

Tension pneumothorax develops when a one-way valve mechanism traps air during inspiration and prevents its escape during expiration, leading to progressive intrathoracic pressure, mediastinal shift, compression of the great vessels, and ultimately hemodynamic collapse. Clinical features include severe dyspnea, hypotension, distended neck veins, tracheal deviation away from the affected side, and absent breath sounds.

Diagnosis is typically confirmed with a chest X-ray, which shows absence of lung markings and visceral pleural line with collapse of lung parenchyma. In tension pneumothorax, diagnosis is clinical, and treatment must not be delayed for imaging. Immediate needle decompression is performed in the 2nd intercostal space at the midclavicular line, followed by placement of a chest tube (tube thoracostomy) in the 5th intercostal space, midaxillary line.

Management of simple pneumothorax depends on size and symptoms. Small, stable primary pneumothoraces may resolve spontaneously with observation and supplemental oxygen, while larger or symptomatic cases require needle aspiration or chest tube placement. Patients with secondary pneumothorax or recurrent episodes may need pleurodesis or surgical intervention to prevent recurrence.

Obstructive Sleep Apnea Syndrome (OSAS) is a common but often underdiagnosed condition characterized by recurrent episodes of upper airway obstruction during sleep, resulting in intermittent hypoxia, sleep fragmentation, and daytime somnolence. It is a high-yield disorder on USMLE Step 2 CK, especially in patients with obesity, loud snoring, witnessed apneas, and daytime fatigue. The pathophysiology involves collapse of the pharyngeal airway due to reduced muscle tone during sleep, typically at the level of the soft palate or oropharynx. Risk factors include obesity, male gender, enlarged tonsils, retrognathia, hypothyroidism, and alcohol or sedative use.

Patients may present with non-restorative sleep, morning headaches, poor concentration, irritability, and nocturia. Bed partners often report loud snoring and gasping or choking episodes during sleep. On physical exam, findings may include obesity, large neck circumference, crowded oropharynx, and hypertension. Untreated OSAS is associated with systemic hypertension, pulmonary hypertension, atrial fibrillation, insulin resistance, and increased risk of stroke and myocardial infarction.

Diagnosis is made via polysomnography (sleep study), which measures the apnea-hypopnea index (AHI). An AHI ≥5 with symptoms, or ≥15 regardless of symptoms, confirms the diagnosis. Overnight oximetry may be suggestive but is not diagnostic. STOP-BANG questionnaire is a useful screening tool in clinical settings.

Management begins with lifestyle modification, including weight loss, avoiding alcohol and sedatives, and sleeping in a lateral position. The mainstay of therapy is continuous positive airway pressure (CPAP), which prevents airway collapse during sleep and improves symptoms and cardiovascular outcomes. In cases of poor CPAP adherence, alternatives include oral mandibular advancement devices or upper airway surgery (e.g., uvulopalatopharyngoplasty). In severe refractory cases, tracheostomy may be considered.

Sarcoidosis is a multisystem granulomatous disease of unknown etiology, characterized by the formation of non-caseating granulomas in affected organs, most commonly the lungs and intrathoracic lymph nodes. It is a high-yield topic on USMLE Step 2 CK, particularly in young adults—especially African American females—presenting with bilateral hilar lymphadenopathy, cough, dyspnea, and constitutional symptoms such as fever, fatigue, and weight loss. Although the lungs are most frequently affected, sarcoidosis can involve virtually any organ system, including the skin, eyes, heart, liver, and nervous system.

Pulmonary manifestations range from asymptomatic radiographic findings to progressive pulmonary fibrosis. Extrapulmonary features include erythema nodosum, lupus pernio, anterior uveitis, dry eyes (keratoconjunctivitis sicca), facial nerve palsy, hepatosplenomegaly, cardiac conduction defects, and hypercalcemia, which occurs due to increased 1-alpha hydroxylase activity in activated macrophages.

Chest X-ray is the initial imaging modality and often shows bilateral hilar lymphadenopathy (Stage I), with or without pulmonary infiltrates (Stage II), or fibrosis (Stage III/IV). High-resolution CT offers more detailed assessment of parenchymal involvement. Serum ACE levels may be elevated but are nonspecific. Hypercalcemia and hypercalciuria are common lab findings. Biopsy of affected tissue, typically lymph nodes or skin lesions, reveals non-caseating granulomas, which is diagnostic after ruling out infections like tuberculosis or fungal disease.

Management depends on severity and organ involvement. Asymptomatic or mild disease may require observation alone, as spontaneous remission is common. Corticosteroids (prednisone) are the mainstay for symptomatic or progressive disease, particularly for pulmonary involvement, ocular disease, cardiac sarcoidosis, or neurologic symptoms. Steroid-sparing agents such as methotrexate or azathioprine may be used in chronic or relapsing cases.