USMLE Step 1 respiratory system questions

An L/S ratio below 2.0 indicates insufficient pulmonary surfactant and predicts a high risk of neonatal respiratory distress syndrome (RDS), which is caused by type II pneumocyte immaturity and inadequate lecithin (phosphatidylcholine) production. Surfactant lowers alveolar surface tension; its deficiency leads to progressive atelectasis, diffuse alveolar damage, and the granular ground-glass appearance with air bronchograms seen on chest x-ray. An L/S ratio ≥2.0 generally indicates fetal lung maturity and low RDS risk. Phosphatidylglycerol appears later in gestation and its presence further supports maturity, the opposite of this infant's clinical picture.

• A. A positive shake (foam stability) test indicates adequate surfactant and predicts fetal lung maturity, not RDS.
• B. Phosphatidylglycerol presence with an L/S ratio of 3.0 reflects mature lungs and would predict a benign respiratory course.
• C. Elevated amniotic alpha-fetoprotein suggests a neural tube defect or abdominal wall defect, not lung immaturity.
• E. An L/S ratio of 2.5 indicates fetal lung maturity and predicts a low risk of RDS, not the course described.

Congenital diaphragmatic hernia, most commonly a left-sided posterolateral (Bochdalek) defect, results from failure of the pleuroperitoneal membrane to close, allowing abdominal organs to herniate into the thoracic cavity early in development. The herniated viscera occupy space normally available for lung growth, mechanically compressing the ipsilateral (and often contralateral) developing lung bud and impairing bronchial branching and alveolarization. This produces pulmonary hypoplasia, the key determinant of postnatal respiratory failure and mortality. The mediastinal shift and bowel-filled hemithorax on imaging are classic findings.

• A. Premature ductal closure causes fetal heart failure and hydrops, not pulmonary hypoplasia from mechanical compression.
• B. This describes tracheoesophageal fistula, which presents with feeding-related choking and polyhydramnios, not a scaphoid abdomen with bowel in the thorax.
• D. This describes the pathogenesis of Hirschsprung disease, unrelated to diaphragmatic development or lung hypoplasia.
• E. This describes truncus arteriosus, a conotruncal cardiac defect unrelated to diaphragmatic herniation.

Bronchopulmonary dysplasia (BPD) results from prolonged exposure of the immature lung to supplemental oxygen and mechanical ventilation (barotrauma/volutrauma) in premature infants. Reactive oxygen species overwhelm the infant's limited antioxidant defenses, triggering inflammation and injury to the developing alveoli and airways. This disrupts normal alveolarization and septation, producing fewer, larger, simplified alveoli and the characteristic patchy, cystic changes on imaging. BPD is diagnosed clinically as oxygen requirement persisting beyond 28 days of life or to 36 weeks postmenstrual age.

• A. Meconium aspiration syndrome occurs in term or post-term infants from in utero meconium passage, not from prolonged ventilation in a preterm infant.
• B. Alpha-1 antitrypsin deficiency causes early-onset emphysema in adults, not neonatal chronic lung disease from oxygen therapy.
• D. Congenital pulmonary lymphangiectasia is a distinct, rare developmental disorder, not the mechanism of ventilator/oxygen-induced lung injury.
• E. BPD is not an antibody-mediated autoimmune process; it results from oxidative and mechanical injury, not maternal antibody transfer.

The most common type of tracheoesophageal anomaly (about 85% of cases) is esophageal atresia with a distal tracheoesophageal fistula connecting the trachea to the distal esophageal segment. The blind proximal esophageal pouch explains the inability to pass a feeding tube and the choking, drooling, and cyanosis with feeding attempts, while polyhydramnios results from the fetus's inability to swallow amniotic fluid in utero. Air is seen in the stomach and bowel because the distal fistula allows air from the trachea to enter the GI tract, distinguishing it from isolated atresia, which shows a gasless abdomen.

• A. Laryngotracheoesophageal cleft is a rare separate anomaly causing aspiration with feeds but does not produce the coiled feeding tube finding of a blind esophageal pouch.
• B. H-type fistula without atresia allows a feeding tube to pass normally into the stomach, which is inconsistent with the resistance described.
• D. Isolated esophageal atresia without a fistula would produce the same tube-coiling finding but a gasless abdomen, since no air can reach the distal bowel.
• E. Proximal fistula with distal atresia is rare and would still produce a gasless abdomen because the distal esophagus is blind-ending with no tracheal communication.

The alveolar gas equation gives PAO2 = FiO2 × (Patm − PH2O) − (PaCO2/R) = 0.21 × (760 − 47) − (30/0.8) ≈ 150 − 37.5 ≈ 112 mm Hg. The A-a gradient is PAO2 − PaO2 ≈ 112 − 65 = 47 mm Hg, markedly elevated above the normal range (roughly 10–15 mm Hg, age-adjusted). Pulmonary embolism obstructs blood flow to ventilated alveoli, creating areas of high V/Q ratio alongside reflex redistribution of flow to other regions, producing overall V/Q mismatch, which widens the A-a gradient because gas exchange becomes inefficient despite adequate ventilation. The patient is also hyperventilating (low PaCO2), which is a compensatory response to hypoxemia and anxiety, not the cause of his hypoxemia.

• B. Diffusion limitation classically causes hypoxemia only during exercise, not at rest, and would still produce an elevated, not normal, A-a gradient.
• C. A congenital right-to-left shunt would have presented much earlier in life and is not suggested by this acute presentation after pulmonary embolism.
• D. Decreased inspired oxygen tension (e.g., high altitude) causes hypoxemia with a normal A-a gradient, not an elevated one as calculated here.
• E. Pure hypoventilation would show an elevated PaCO2 and a normal A-a gradient; this patient has a low PaCO2 and a markedly elevated gradient.

The Bohr effect describes how increased H+ (from increased CO2 and lactic acid production during exercise) binds to specific residues on hemoglobin, favoring and stabilizing the deoxygenated taut (T) conformation over the oxygenated relaxed (R) conformation. The T state has lower affinity for oxygen, shifting the dissociation curve to the right and promoting oxygen unloading at the tissue level where it is metabolically needed. Increased temperature and 2,3-BPG act through related allosteric mechanisms that also favor the T state and rightward shift. This is physiologically advantageous because actively metabolizing tissue needs more oxygen delivered exactly where CO2 and acid are being produced.

• A. Carbon dioxide affects oxygen binding mainly through carbamino formation and pH-driven conformational change, not by directly competing with oxygen at the heme iron.
• C. 2,3-BPG binds the beta-globin chains, not the alpha chains, and its effect is to decrease, not enhance, oxygen affinity.
• D. 2,3-BPG binds preferentially to the deoxygenated (taut) form, not the oxygenated form, and decreases, not increases, oxygen affinity.
• E. Increased temperature does not change the number of binding sites; it decreases hemoglobin's oxygen affinity by promoting the taut conformation.

Hypoxic pulmonary vasoconstriction is a unique, local, non-neural response in which alveolar hypoxia causes contraction of pulmonary vascular smooth muscle in the arterioles supplying that specific region. This diverts blood flow away from poorly ventilated alveoli (as in atelectasis) toward better-ventilated regions, optimizing overall ventilation-perfusion matching. The mechanism involves hypoxia-induced inhibition of oxygen-sensitive potassium channels in pulmonary vascular smooth muscle, leading to membrane depolarization, calcium influx, and contraction. This response is the opposite of the vasodilation that hypoxia produces in most other systemic vascular beds.

• A. It is alveolar hypoxia, not hypocapnia, that is the primary trigger for pulmonary vasoconstriction in this setting.
• B. Nitric oxide is a vasodilator, not a vasoconstrictor, and its release is generally decreased, not increased, in hypoxic lung regions.
• C. Prostacyclin is a vasodilator produced by endothelium and would oppose, not promote, vasoconstriction.
• D. Hypoxic pulmonary vasoconstriction is a local, intrinsic smooth muscle response independent of systemic sympathetic innervation.

West zone 2 occurs in the mid-lung and is characterized by pulmonary arterial pressure exceeding alveolar pressure, which in turn exceeds pulmonary venous pressure. Because alveolar pressure is higher than venous pressure, flow is governed by the arterial-to-alveolar pressure gradient rather than the usual arterial-to-venous gradient, and venous pressure changes do not affect flow (a Starling resistor effect). This contrasts with zone 1 (alveolar pressure exceeds both vascular pressures, causing no flow, seen in hypovolemia or positive-pressure ventilation) and zone 3 (arterial and venous pressures both exceed alveolar pressure, so flow depends on the normal arteriovenous gradient, as at the lung bases).

• A. This relationship does not correspond to any physiologic West zone.
• B. This arterial > venous > alveolar relationship defines zone 3 (lung base), where flow is governed by the normal arteriovenous gradient.
• D. This defines zone 1 (lung apex under low cardiac output or positive pressure ventilation), where alveolar pressure exceeds both vascular pressures and flow ceases.
• E. Venous pressure does not normally exceed arterial pressure in the upright lung.

CO2 crosses the blood-brain barrier easily and enters the cerebrospinal fluid, where carbonic anhydrase catalyzes its hydration to carbonic acid, which dissociates into H+ and bicarbonate. The resulting fall in CSF pH stimulates central chemoreceptors on the ventral medulla, producing a rapid and robust increase in ventilation; this is the dominant drive for minute-to-minute ventilatory control under normal conditions. Peripheral chemoreceptors (carotid and aortic bodies) respond primarily to hypoxemia (PO2 less than approximately 60 mm Hg) and contribute only a minor component of the CO2 response. Central chemoreceptors do not directly sense oxygen, which is why hypoxic drive is a comparatively weaker and slower stimulus.

• A. CO2's main ventilatory effect is mediated through central chemoreceptors via CSF pH, not through a pH-independent peripheral carotid body mechanism.
• B. Hypoxia does not raise CSF bicarbonate as its stimulatory mechanism; hypoxic sensing occurs in peripheral carotid bodies through different ion channel mechanisms.
• C. Carotid body hypoxia sensing (not CO2 sensing) involves inhibition of oxygen-sensitive potassium channels in glomus cells, not calcium channel effects from CO2.
• D. Central chemoreceptors are not directly sensitive to oxygen tension; hypoxia is sensed mainly by peripheral carotid body chemoreceptors.

In patients with chronic CO2 retention, central chemoreceptors become relatively desensitized to CO2 over time, and ventilation becomes more dependent on hypoxic stimulation of peripheral chemoreceptors in the carotid bodies. Administering high-concentration supplemental oxygen eliminates this hypoxic drive, reducing minute ventilation and causing progressive CO2 retention (hypercapnia) and worsening respiratory acidosis, which manifests clinically as somnolence or CO2 narcosis. Additional contributing mechanisms include worsening ventilation-perfusion mismatch (reversal of hypoxic vasoconstriction) and the Haldane effect, but blunting of hypoxic ventilatory drive is the classically tested mechanism. Oxygen should be titrated carefully in chronic CO2 retainers to a target saturation around 88-92% rather than withheld entirely.

• A. High-flow oxygen is not known to precipitate bronchospasm as the mechanism of hypercapnic somnolence in this scenario.
• B. Oxygen-induced acute pulmonary edema is not a recognized mechanism of hypercapnia in this setting.
• C. Oxygen toxicity from acute high-concentration therapy causing direct medullary suppression is not the mechanism of this phenomenon.
• D. Hyperoxia typically decreases, not increases, cerebral blood flow, and this is not the mechanism of CO2 narcosis.

At high altitude, the hypoxia-driven hyperventilation (via peripheral chemoreceptor stimulation) causes a primary respiratory alkalosis, with low PCO2 and a compensatory rise in pH. Over several days, the kidneys compensate by decreasing bicarbonate reabsorption in the proximal tubule and increasing bicarbonate excretion in the urine, lowering plasma bicarbonate toward normal and returning pH closer to 7.40. This renal compensation is slower than the respiratory response (taking days) but allows sustained hyperventilation and improved oxygen-carrying capacity without severe alkalemia, an important part of acclimatization along with increased 2,3-BPG and erythropoietin-driven erythropoiesis.

• A. This describes compensation for a primary metabolic acidosis, which is the opposite of the alkalosis present here.
• B. Increased hydrogen secretion would raise, not lower, plasma bicarbonate, which is the opposite of the renal response seen in respiratory alkalosis.
• C. Decreased carbonic anhydrase activity and increased bicarbonate reabsorption would raise plasma bicarbonate, opposite to what occurs in appropriate renal compensation for alkalosis.
• E. Aldosterone-driven bicarbonate generation is not the primary mechanism of renal compensation for high-altitude respiratory alkalosis.

Chronic bronchitis is a clinical diagnosis defined by a productive cough for at least 3 months in 2 consecutive years, and its histologic correlate is hypertrophy and hyperplasia of the submucosal mucus-secreting glands in the bronchial wall, quantified by the Reid index. A Reid index greater than 0.4 (here, 0.6) confirms the glandular hyperplasia characteristic of chronic bronchitis, reflecting the chronic irritation and mucus hypersecretion driven by cigarette smoke exposure. This contrasts with emphysema, whose hallmark is destruction of alveolar walls, and with asthma, whose sputum shows eosinophilic markers such as Curschmann spirals and Charcot-Leyden crystals.

• A. Caseating granulomas suggest tuberculosis or fungal infection, not chronic bronchitis.
• B. This describes the pathology of emphysema, not chronic bronchitis.
• C. A Reid index of 0.3 falls within the normal range and would not support the diagnosis of chronic bronchitis.
• D. These findings are characteristic of asthma (eosinophilic airway inflammation), not chronic bronchitis.

Centriacinar (centrilobular) emphysema, the pattern seen in smokers, predominantly affects the upper lobes because inhaled cigarette smoke particulates and the resulting neutrophil and macrophage infiltration are concentrated in the respiratory bronchioles, the proximal part of the acinus. Neutrophil elastase released by these inflammatory cells destroys the walls of the respiratory bronchioles and adjacent alveoli while sparing the distal alveolar ducts and sacs. This contrasts with panacinar emphysema in alpha-1-antitrypsin deficiency, which uniformly destroys the entire acinus and predominates in the lower lobes because unopposed elastase activity is not anatomically restricted. The distribution difference reflects where the inciting insult (smoke deposition) versus the underlying defect (systemic antiprotease deficiency) exerts its effect.

• A. Perfusion gradients do not determine antiprotease delivery or emphysema distribution; the mechanism is proximal acinar inflammation from inhaled smoke.
• C. Alpha-1-antitrypsin is synthesized in the liver, not locally by lower lobe macrophages.
• D. Ventilation-perfusion ratios do not account for the specific centriacinar distribution pattern seen in smoking-related emphysema.
• E. Pneumocyte type ratios do not differ significantly by lobe in a way that explains emphysema distribution.

Alpha-1-antitrypsin (A1AT) is synthesized by hepatocytes and normally inhibits neutrophil elastase in the lung. The Z allele produces a misfolded A1AT protein that polymerizes and accumulates within the endoplasmic reticulum of hepatocytes, forming PAS-positive, diastase-resistant globules and causing hepatocyte injury that can progress to cirrhosis. Because the mutant protein cannot be properly secreted, serum and alveolar A1AT levels are low, leaving neutrophil elastase unopposed in the lung and causing panacinar emphysema, especially in the lower lobes. The PiZZ genotype represents homozygosity for the Z allele and carries the highest risk of both severe emphysema and liver disease. This is a classic example of a gain-of-function toxic accumulation causing one organ's pathology and a loss-of-function deficiency causing another's.

• A. This describes the CFTR mutation of cystic fibrosis, not alpha-1-antitrypsin deficiency.
• B. The liver does not release elastase; hepatic injury results from intracellular protein accumulation, not elastase secretion.
• C. This describes Wilson disease, not alpha-1-antitrypsin deficiency.
• D. There is no autoantibody-mediated mechanism in alpha-1-antitrypsin deficiency.

Asthma exacerbations following allergen exposure classically occur in two phases. The early phase occurs within minutes and results from allergen cross-linking IgE bound to mast cells, triggering degranulation and release of preformed histamine and leukotrienes, causing immediate bronchospasm. The late phase occurs 4 to 8 hours later and is driven by recruitment of eosinophils, along with Th2 lymphocytes and basophils, which release major basic protein, eosinophil cationic protein, and additional leukotrienes (especially LTC4, LTD4, LTE4), causing sustained bronchoconstriction, mucus hypersecretion, and airway edema. Repeated late-phase inflammation over time contributes to airway remodeling in chronic asthma. Inhaled corticosteroids primarily target this eosinophilic late-phase inflammation.

• A. CD8+ cytotoxic T lymphocytes are more associated with viral infections and are not the predominant late-phase effector cell in allergic asthma.
• B. Plasma cells are not characteristically recruited during the late-phase response; IgE is produced earlier by sensitized B cells.
• D. Natural killer cells are not the predominant effector cell recruited in the late-phase asthmatic response.
• E. Neutrophils are more characteristic of bacterial infection or severe status asthmaticus, not the typical allergic late-phase response.

Albuterol is a short-acting beta-2 adrenergic receptor agonist. Beta-2 receptors are coupled to Gs proteins, and their activation stimulates adenylyl cyclase to convert ATP to cyclic AMP. Increased intracellular cAMP activates protein kinase A, which phosphorylates and inhibits myosin light-chain kinase while promoting myosin light-chain phosphatase activity, ultimately lowering intracellular calcium sensitivity and causing bronchial smooth muscle relaxation. This mechanism provides rapid relief of bronchospasm in acute asthma exacerbations. Beta-2 agonists also decrease mast cell mediator release, though their primary therapeutic effect is direct bronchial smooth muscle relaxation.

• A. Beta-2 agonists increase cAMP via adenylyl cyclase, not cGMP via phosphodiesterase inhibition; phosphodiesterase-4 inhibitors like roflumilast act through a different pathway.
• B. This describes the mechanism of leukotriene receptor antagonists such as montelukast, not beta-2 agonists.
• C. Albuterol does not act presynaptically on cholinergic nerve terminals; that mechanism describes anticholinergics like ipratropium acting postsynaptically, not beta agonists.
• D. This describes Gq-coupled receptor signaling such as that of muscarinic M3 or alpha-1 receptors, which promotes smooth muscle contraction, not relaxation.

This patient has oral candidiasis (thrush), a well-recognized local adverse effect of inhaled corticosteroids such as fluticasone. Corticosteroid deposition in the oropharynx suppresses local mucosal immune defenses and promotes overgrowth of Candida albicans. Rinsing the mouth with water and spitting it out immediately after each inhaler use removes residual drug from the oropharyngeal mucosa and significantly reduces the risk of candidiasis. Using a spacer device with metered-dose inhalers also reduces oropharyngeal deposition. This simple behavioral measure is standard counseling given to all patients prescribed inhaled corticosteroids.

• A. Switching to systemic corticosteroids would increase systemic adverse effects and is not an appropriate substitution for an inhaled form to prevent a local adverse effect.
• B. Using a beta-agonist first does not reduce oropharyngeal corticosteroid deposition or candidiasis risk.
• C. Timing relative to tooth brushing has no established role in preventing corticosteroid-associated oral candidiasis.
• D. Inhaled corticosteroids act locally in the airway; taking them with food does not reduce oropharyngeal deposition or candidiasis risk.

This patient has aspirin-exacerbated respiratory disease (Samter triad: asthma, nasal polyps, and aspirin/NSAID sensitivity). COX inhibition by aspirin or NSAIDs shunts arachidonic acid metabolism toward the 5-lipoxygenase pathway, markedly increasing production of cysteinyl leukotrienes (LTC4, LTD4, LTE4), which cause bronchoconstriction, mucus secretion, and vascular permeability. Montelukast is a leukotriene receptor antagonist that selectively blocks the cysteinyl leukotriene type 1 (CysLT1) receptor, preventing these mediators from exerting their effects and providing effective prophylaxis in this condition. Zileuton, an alternative agent, instead inhibits 5-lipoxygenase directly to prevent leukotriene synthesis.

• A. H1 receptor blockade targets histamine-mediated allergic symptoms, not leukotriene-mediated bronchoconstriction in this condition.
• B. Further COX-2 inhibition would not help and may still precipitate symptoms in aspirin-sensitive patients since the problem is excess leukotriene production, not insufficient COX inhibition.
• C. This describes phosphodiesterase-4 inhibitors like roflumilast, used in COPD, not leukotriene receptor antagonists.
• E. This describes mast cell stabilizers like cromolyn, which act upstream of leukotriene release rather than blocking the leukotriene receptor.

Bronchiectasis is permanent dilation of bronchi and bronchioles caused by chronic inflammation and destruction of the muscular and elastic components of the bronchial wall, most often from recurrent or severe infection. Causes include cystic fibrosis, recurrent bacterial infections, primary ciliary dyskinesia, and immunodeficiency. Histologically, the airway wall shows chronic inflammatory infiltrate, destruction of the normal bronchial architecture, and fibrous scarring that replaces the respiratory epithelium, with marked bronchial dilation apparent on imaging. This patient's normal sweat chloride makes cystic fibrosis unlikely, pointing toward another cause of recurrent infection such as primary ciliary dyskinesia, but the underlying histologic destruction and fibrosis pattern is shared across all causes of bronchiectasis.

• A. Non-caseating granulomas in a lymphatic distribution are characteristic of sarcoidosis.
• B. This describes allergic bronchopulmonary aspergillosis with fungal invasion, which can cause bronchiectasis but is not the general histologic finding and this patient lacks eosinophilic/allergic features described here.
• D. Diffuse alveolar damage with hyaline membranes describes acute respiratory distress syndrome, not bronchiectasis.
• E. Caseating granulomas with Langhans giant cells are characteristic of tuberculosis, not simple bronchiectasis.

This infant has classic features of cystic fibrosis, caused here by the ΔF508 mutation, the most common CFTR mutation, which results in a misfolded chloride channel protein that is degraded by the proteasome before reaching the apical membrane of epithelial cells. Loss of functional CFTR impairs chloride (and secondarily sodium and water) transport, producing thick, dehydrated secretions in the lungs and pancreas and abnormally high chloride concentration in sweat due to defective chloride reabsorption in sweat ducts. The pilocarpine iontophoresis sweat chloride test is the gold-standard confirmatory diagnostic test, with a sweat chloride concentration greater than 60 mEq/L on two occasions being diagnostic. Newborn screening measures immunoreactive trypsinogen, but an elevated level only prompts further testing and is not itself confirmatory.

• A. Immunoreactive trypsinogen is used for newborn screening, not confirmatory diagnosis; it can be elevated in other neonatal conditions as well.
• B. Nasal potential difference can support the diagnosis in equivocal cases but is not the standard first-line confirmatory test.
• C. Alpha-1-antitrypsin level evaluates for a different genetic disease causing emphysema and liver disease, unrelated to this presentation.
• E. Fecal elastase assesses pancreatic exocrine function and supports a diagnosis of pancreatic insufficiency but does not confirm cystic fibrosis itself.

A reduced FEV1/FVC ratio below the predicted lower limit (here 56%, well under the ~70% threshold) indicates airflow obstruction, since expiratory flow is disproportionately reduced relative to total volume exhaled. The increased total lung capacity reflects air trapping from small airway collapse and loss of elastic recoil, classic for emphysema/COPD. In restrictive disease, both FEV1 and FVC fall proportionally, keeping the FEV1/FVC ratio normal or increased, and total lung capacity is reduced rather than increased. This patient's smoking history and hyperinflation support an obstructive process.

• A. The abnormal FEV1/FVC ratio and increased TLC indicate true pathology, not poor effort.
• B. There is no evidence of a superimposed restrictive component such as reduced TLC.
• C. Restrictive patterns show decreased TLC, and diffusion capacity is not assessed by this data.
• D. Restrictive disease would show reduced, not increased, total lung capacity.

In sarcoidosis, activated macrophages within noncaseating granulomas express 1-alpha-hydroxylase, which converts 25-hydroxyvitamin D into the active form, 1,25-dihydroxyvitamin D (calcitriol), independent of renal and parathyroid regulation. Calcitriol increases intestinal calcium absorption and bone resorption, leading to hypercalcemia and hypercalciuria. This mechanism explains why hypercalcemia in sarcoidosis can worsen with sun exposure or vitamin D supplementation and improves with glucocorticoid therapy, which suppresses macrophage activity. ACE elevation reflects granuloma burden but does not cause hypercalcemia.

• A. Granulomas do not produce parathyroid hormone; sarcoidosis-related hypercalcemia is PTH-independent and typically suppresses endogenous PTH.
• B. PTHrP-mediated hypercalcemia is characteristic of certain malignancies (e.g., squamous cell lung cancer), not sarcoidosis.
• D. There is no evidence of malignancy or bone metastases in this presentation.
• E. While sarcoidosis can cause nephrocalcinosis, decreased renal excretion is not the primary mechanism of hypercalcemia here.

Silicosis results from chronic inhalation of crystalline silica particles, common in sandblasting, mining, and quarrying. It characteristically produces upper lobe-predominant nodular opacities and eggshell calcification of hilar lymph nodes, a peripheral rim of calcification around enlarged nodes, which is highly specific for this disease. Silica particles are toxic to macrophages, causing cell death and release of inflammatory mediators that promote fibrosis, and silicosis also increases susceptibility to tuberculosis by impairing macrophage function. Coal worker's pneumoconiosis and asbestosis have different radiographic patterns and occupational exposures.

• A. Berylliosis resembles sarcoidosis with noncaseating granulomas, not eggshell calcification, and is linked to aerospace or electronics exposure.
• B. Hypersensitivity pneumonitis follows organic antigen exposure, not mineral dust, and does not cause eggshell calcification.
• C. Coal worker's pneumoconiosis causes diffuse small nodular opacities but not the eggshell calcification seen with silica exposure.
• D. Asbestosis typically causes lower lobe-predominant fibrosis with pleural plaques, not eggshell calcification or sandblasting exposure.

Malignant mesothelioma arises from the mesothelial lining of the pleura and is strongly associated with asbestos exposure, with a long latency period often exceeding 20-40 years. It characteristically presents with diffuse pleural thickening, nodularity, and effusion rather than a discrete parenchymal mass, and the biphasic epithelioid-sarcomatoid histology with positive calretinin and WT-1 immunostaining is diagnostic, distinguishing it from pulmonary adenocarcinoma, which stains positive for TTF-1 and napsin A. Asbestos exposure also increases the risk of bronchogenic carcinoma, and the combination of smoking with asbestos exposure synergistically raises this risk, but the diffuse pleural pattern and immunoprofile here point to mesothelioma.

• B. Small cell carcinoma arises centrally and shows small cells with scant cytoplasm and neuroendocrine markers, not this biphasic pattern.
• C. Renal cell carcinoma metastases would show clear cell or papillary histology and a renal primary, not calretinin/WT-1 positive biphasic tumor.
• D. Adenocarcinoma would typically show a discrete pulmonary mass and stain positive for TTF-1, not calretinin and WT-1.
• E. Squamous cell carcinoma usually presents as a central lung mass with keratinization on histology, not diffuse pleural disease.

This patient has multiple components of Virchow's triad: venous stasis from recent immobilization after surgery, hypercoagulability from malignancy and tamoxifen use, and likely endothelial injury from surgery, placing her at high risk for deep venous thrombosis and pulmonary embolism. The combination of sudden dyspnea, pleuritic chest pain, tachycardia, hypoxemia, and a swollen tender calf (suggesting DVT as the embolic source) is the classic presentation of acute PE. Lung auscultation is often normal or shows only mild findings in PE, unlike pneumonia or asthma. The clinical probability here (recent surgery, immobilization, malignancy, unilateral leg swelling) strongly favors thromboembolism over the other diagnoses.

• B. Pneumothorax classically causes unilateral absent breath sounds and hyperresonance, not present in this patient.
• C. Pneumonia typically presents with productive cough, fever, and focal consolidation on exam, which are absent here.
• D. Asthma exacerbation presents with wheezing and a history of reactive airway disease, not reflected in this vignette.
• E. MI would more likely present with substernal chest pressure and ECG/enzyme changes rather than pleuritic pain and unilateral leg swelling.

Fat embolism syndrome classically follows long bone or pelvic fractures, presenting with the triad of respiratory distress, neurologic changes, and petechial rash, typically 24-72 hours after injury. Marrow fat is released into the venous circulation and travels to the pulmonary capillaries, causing both mechanical obstruction and local toxic injury as fat is hydrolyzed into free fatty acids by lipase, damaging capillary endothelium and inducing an inflammatory response. This produces capillary leak, alveolar hemorrhage, and the characteristic petechiae from microvascular occlusion and thrombocytopenia. The timing and petechial rash distinguish this from pulmonary thromboembolism, which presents acutely and without rash.

• A. Immune complex-mediated lung injury is not the mechanism in fat embolism syndrome.
• B. There is no evidence of infection such as fever with positive cultures; this is a sterile inflammatory process.
• C. Disseminated intravascular coagulation can complicate severe trauma but does not primarily explain the fat-induced capillary injury.
• D. Paradoxical embolism requires a right-to-left shunt and typically presents with systemic arterial occlusion, not this petechial rash pattern.

Amniotic fluid embolism is a peripartum catastrophe characterized by sudden cardiovascular collapse, respiratory failure, altered mental status or seizures, and disseminated intravascular coagulation, typically occurring during labor, delivery, or shortly after. Entry of amniotic fluid and fetal debris into the maternal circulation triggers a massive systemic inflammatory and anaphylactoid-like response, activating coagulation and causing profound DIC with hypofibrinogenemia and thrombocytopenia, as seen in this patient's labs. The rapid, dramatic onset with hypoxemia, hypotension, seizure, and consumptive coagulopathy distinguishes it from isolated postpartum hemorrhage or cardiomyopathy. Diagnosis is clinical, as there is no confirmatory laboratory test.

• A. Peripartum cardiomyopathy develops over days to months postpartum with gradual heart failure symptoms, not this abrupt collapse.
• B. Eclampsia presents with hypertension and seizures but does not typically cause this degree of hypoxemia and DIC immediately after delivery.
• C. Uterine atony causes hemorrhage but does not explain the preceding respiratory failure, seizure, and consumptive coagulopathy.
• E. Pulmonary thromboembolism causes hypoxemia and hypotension but does not explain the seizure and severe consumptive coagulopathy pattern.

This patient has heritable pulmonary arterial hypertension due to a BMPR2 mutation, the most common genetic cause of familial PAH. BMPR2 normally signals through the TGF-beta superfamily pathway to inhibit proliferation of pulmonary vascular smooth muscle and endothelial cells. Loss-of-function mutations in BMPR2 remove this growth-inhibitory signal, leading to uncontrolled proliferation, vascular remodeling, medial hypertrophy, and the pathognomonic plexiform lesions seen on biopsy. This results in progressively elevated pulmonary vascular resistance and right ventricular pressure overload, producing the loud P2 and parasternal heave on exam, with a normal wedge pressure confirming a precapillary (arterial) process rather than left heart disease.

• A. BMPR2 loss does not increase nitric oxide synthase activity; endothelial dysfunction in PAH actually reduces nitric oxide bioavailability.
• B. Prostacyclin synthesis is reduced in PAH, promoting vasoconstriction and remodeling rather than its regression.
• C. Endothelin-1 is typically increased, not decreased, in pulmonary arterial hypertension, contributing to vasoconstriction.
• E. Potassium channel dysfunction in PAH leads to membrane depolarization and vasoconstriction, not hyperpolarization.

Cor pulmonale is right ventricular hypertrophy, dilation, and eventual failure resulting from a primary lung disorder, most commonly COPD. Chronic alveolar hypoxia in COPD triggers hypoxic pulmonary vasoconstriction, a normally adaptive mechanism that becomes pathologic when sustained and diffuse, leading to chronically elevated pulmonary vascular resistance and pulmonary arterial pressure. This increases right ventricular afterload, causing right ventricular hypertrophy and, eventually, dilation and failure with systemic venous congestion (jugular venous distension, hepatomegaly, peripheral edema). The preserved left ventricular ejection fraction confirms that the right heart failure is secondary to pulmonary disease rather than left heart pathology.

• A. Cor pulmonale from COPD is a hypoxia-driven vascular and mechanical process, not an autoimmune endothelial process.
• C. Cigarette smoke primarily damages lung parenchyma and airways; it does not directly cause right ventricular myocyte toxicity in cor pulmonale.
• D. Bronchial-pulmonary shunting is not the mechanism of cor pulmonale and does not explain the elevated pulmonary vascular resistance.
• E. Preserved ejection fraction without evidence of left heart disease argues against left ventricular diastolic dysfunction as the cause.

Unfractionated heparin binds to and activates antithrombin III, which then inactivates thrombin (factor IIa) and factor Xa; its anticoagulant effect is monitored using the partial thromboplastin time (PTT), with the dose titrated to a therapeutic range. Warfarin inhibits vitamin K epoxide reductase, blocking the gamma-carboxylation and activation of vitamin K-dependent clotting factors II, VII, IX, and X (and proteins C and S); its effect is monitored using the prothrombin time, reported as the international normalized ratio (PT/INR). Heparin and warfarin are overlapped initially because warfarin transiently depletes protein C before fully reducing procoagulant factors, creating a brief hypercoagulable window; heparin is continued until the INR is therapeutic to prevent this paradoxical thrombosis risk.

• B. Heparin does not directly inhibit factor Xa (that describes agents like fondaparinux or direct Xa inhibitors), and warfarin does not activate protein C as its therapeutic mechanism.
• C. This reverses the mechanisms and monitoring tests of heparin and warfarin.
• D. Warfarin inhibits synthesis of vitamin K-dependent factors, not antithrombin III, and PT/INR, not PTT, is used for warfarin monitoring.
• E. Heparin's primary anticoagulant mechanism is antithrombin activation, not platelet inhibition, and bleeding time is not used to monitor heparin.

Light's criteria classify a pleural effusion as exudative if any one of the following is present: pleural fluid to serum protein ratio greater than 0.5, pleural fluid to serum LDH ratio greater than 0.6, or pleural fluid LDH greater than two-thirds the upper limit of normal serum LDH. In this patient, the pleural fluid to serum protein ratio is 4.5/7.2 = 0.625, which exceeds 0.5, meeting criteria for an exudate on this basis alone; only one criterion needs to be met to classify the effusion as exudative. This is consistent with a malignant effusion from his lung carcinoma, since exudates result from increased capillary permeability or impaired lymphatic drainage, in contrast to transudates caused by increased hydrostatic or decreased oncotic pressure as in isolated heart failure.

• A. The calculated protein ratio of 0.625 exceeds 0.5, meeting criteria for an exudate, not a transudate.
• B. The pleural fluid to serum LDH ratio here is 260/180 = 1.44, which is greater than 0.6, also supporting exudate, not contradicting it via this reasoning, making this statement's criterion value incorrect.
• C. Only one of Light's three criteria needs to be met to classify an effusion as exudative; this effusion meets at least one (and here, more than one) criterion, so it is a definitive exudate, not indeterminate.
• E. This statement inverts the criterion; an LDH less than two-thirds the upper limit of normal would support a transudate, but the protein ratio alone already confirms an exudate.

Primary spontaneous pneumothorax occurs classically in tall, thin young men due to rupture of apical subpleural blebs, which are small air-filled sacs thought to arise from increased mechanical stress and reduced perfusion at the lung apex. Air leaks from the alveolus into the pleural space, collapsing the lung and causing ipsilateral hyperresonance and decreased breath sounds with a midline trachea (simple, not tension, pneumothorax). Risk factors include smoking, male sex, and a tall, thin body habitus (Marfan-like thoracic configuration). This differs from secondary spontaneous pneumothorax, which occurs in patients with underlying lung disease such as COPD or cystic lung disease.

• A. Bronchogenic cysts are rare congenital lesions and not the typical cause in this demographic.
• C. No trauma history is given, and rib fracture is a cause of traumatic, not spontaneous, pneumothorax.
• D. There is no abdominal pathology or peritoneal signs suggesting a perforated viscus.
• E. There is no evidence of malignancy in this young patient with an acute, isolated event.

This patient has tension pneumothorax, a clinical diagnosis based on respiratory distress, hypotension, tracheal deviation away from the affected side, distended neck veins, and ipsilateral absent breath sounds with hyperresonance. It is a life-threatening emergency caused by accumulating pleural air that compresses the lung and shifts the mediastinum, kinking the vena cava and reducing venous return, which causes obstructive shock. Immediate needle decompression at the second intercostal space, midclavicular line, converts the tension pneumothorax to a simple one and must precede imaging. Definitive management with chest tube thoracostomy follows decompression.

• A. Imaging delays life-saving treatment in a patient with a clinical diagnosis and hemodynamic instability.
• B. Positive pressure ventilation can worsen tension pneumothorax before decompression and does not address the underlying problem.
• C. CT delays treatment and is contraindicated in an unstable patient; it is not the immediate step.
• E. The findings indicate pneumothorax, not cardiac tamponade, which would show muffled heart sounds and no tracheal deviation.

Empyema is frank pus or grossly infected fluid in the pleural space, diagnosed by pleural fluid with low pH (<7.20), low glucose (<60 mg/dL), high LDH, a markedly elevated neutrophil-predominant white cell count, and a positive Gram stain or culture, as seen here following untreated or inadequately treated bacterial pneumonia. The low pH and glucose reflect high metabolic activity of neutrophils and bacteria consuming glucose and producing acid. These findings meet Light's criteria for an exudate and indicate the need for tube thoracostomy drainage in addition to antibiotics, since antibiotics alone are insufficient once frank pus has formed. Untreated empyema can progress to a fibrothorax with a thickened pleural rind.

• B. Malignant effusions are typically exudative with a positive cytology, not a positive bacterial culture with markedly low pH and glucose.
• C. Chylothorax shows milky fluid with high triglycerides, not purulent fluid with positive bacterial culture.
• D. Hemothorax is diagnosed by a pleural fluid hematocrit at least half of peripheral blood hematocrit, not by positive bacterial culture.
• E. Heart failure produces a transudate with normal pH, normal glucose, low LDH, and a negative culture.

Malignant mesothelioma characteristically stains positive for calretinin, WT-1, and cytokeratin 5/6, and is negative for carcinoembryonic antigen (CEA), while metastatic pulmonary adenocarcinoma shows the opposite pattern (CEA and Ber-EP4 positive, calretinin negative). This immunophenotypic panel is essential because mesothelioma and adenocarcinoma can appear histologically similar, including papillary architecture, but have very different prognoses and treatments. Mesothelioma arises from mesothelial cells lining the pleura and is strongly associated with asbestos exposure after a long latency period. TTF-1 and napsin A are markers of pulmonary adenocarcinoma, not mesothelioma.

• A. S-100 and HMB-45 are melanoma markers and are not relevant to this differential.
• B. This CEA/Ber-EP4-positive, calretinin-negative pattern is characteristic of adenocarcinoma, not mesothelioma.
• C. CK7+/CK20- is a nonspecific pattern seen in many adenocarcinomas including lung, and does not distinguish mesothelioma from adenocarcinoma.
• D. TTF-1 and napsin A positivity indicates pulmonary adenocarcinoma, not mesothelioma.

Streptococcus pneumoniae is the most common cause of community-acquired typical (lobar) pneumonia, characteristically presenting with acute fever, productive rust-colored or blood-tinged sputum, and lobar consolidation on imaging. Its polysaccharide capsule is the main virulence factor, enabling evasion of phagocytosis, and is the target of pneumococcal vaccines. The Gram stain finding of gram-positive, lancet-shaped (pointed) diplococci in pairs is classic and distinguishes it from other gram-positive cocci. Lobar consolidation reflects alveolar filling with fibrin-rich exudate that spreads contiguously through the lobe via the pores of Kohn.

• A. Staphylococcus aureus appears as gram-positive cocci in clusters, not lancet-shaped diplococci in pairs.
• B. Klebsiella causes lobar pneumonia with currant jelly sputum in alcoholics but appears as a gram-negative encapsulated rod, not a gram-positive diplococcus.
• C. Haemophilus influenzae is a small gram-negative coccobacillus, not a gram-positive diplococcus.
• D. Mycoplasma is a gram-indeterminate organism lacking a cell wall and causes an atypical, interstitial pattern, not lobar consolidation.

Mycoplasma pneumoniae infection triggers production of IgM autoantibodies directed against the I antigen on red blood cells; these cold agglutinins bind erythrocytes at low temperatures in peripheral circulation and fix complement, causing intravascular hemolysis and agglutination that reverses with warming. This is thought to result from cross-reactivity or polyclonal B-cell activation following infection and classically accompanies the atypical, interstitial pneumonia pattern with a dry cough and exam findings milder than the chest x-ray suggests. Mycoplasma lacks a cell wall, so it cannot be seen on Gram stain, and it is diagnosed serologically or by PCR. The clinical mismatch between radiographic severity and mild examination findings is a classic clue to atypical pneumonia.

• A. Autoantibodies to erythropoietin receptors are not a recognized mechanism of Mycoplasma-associated anemia.
• B. There is no evidence of schistocytes, thrombocytopenia, or organ ischemia to suggest a microangiopathic process.
• C. Mycoplasma does not directly invade or lyse erythrocytes; its hemolysis is immune-mediated.
• E. This describes warm autoimmune hemolytic anemia, which is IgG-mediated and not typically cold-reactive or linked to Mycoplasma.

Legionella pneumophila is a gram-negative intracellular organism that stains poorly with standard Gram stain, requiring silver stain or special culture media (buffered charcoal yeast extract agar) for direct visualization. The Legionella urinary antigen test is the most rapid and widely used diagnostic method in clinical practice, detecting a serogroup 1 antigen with high sensitivity and specificity, and remains positive even after antibiotics are started. The clinical picture of severe pneumonia with extrapulmonary features, including hyponatremia, gastrointestinal symptoms, confusion, and elevated liver enzymes, strongly suggests Legionella, which is acquired from contaminated water sources such as air conditioning systems, hot tubs, or fountains rather than person-to-person transmission.

• A. Legionella grows only on specialized buffered charcoal yeast extract agar, not standard blood agar.
• B. Galactomannan is a marker for invasive aspergillosis, not Legionella infection.
• C. Cold agglutinins are associated with Mycoplasma pneumoniae, not Legionella.
• E. Giemsa stain is used for organisms such as Pneumocystis or blood parasites, not Legionella.

Klebsiella pneumoniae possesses a thick polysaccharide capsule that confers the characteristic mucoid colony morphology on culture and produces the thick, viscous, currant jelly sputum seen clinically. This capsule also impairs phagocytosis and contributes to the organism's virulence and propensity to cause necrotizing lobar pneumonia with cavitation, classically affecting the upper lobes in patients with alcohol use disorder or diabetes mellitus who have impaired airway protective reflexes and aspiration risk. The gelatinous, blood-tinged sputum results from capsule material mixed with necrotic lung tissue and blood. Klebsiella is a lactose-fermenting gram-negative rod of the Enterobacteriaceae family.

• A. Exotoxin A is a virulence factor of Pseudomonas aeruginosa, not Klebsiella.
• B. Pyocyanin is a blue-green pigment produced by Pseudomonas aeruginosa, not Klebsiella.
• C. While Klebsiella has lipopolysaccharide (endotoxin) contributing to systemic illness, it is not responsible for the mucoid colony or sputum appearance.
• E. IgA protease is a virulence factor of mucosal pathogens like Streptococcus pneumoniae and Haemophilus influenzae, not primarily responsible for Klebsiella's mucoid appearance.

Patients with HIV and a CD4+ count below 200/mm³ are at high risk for Pneumocystis jirovecii pneumonia and require prophylaxis with trimethoprim-sulfamethoxazole, which inhibits sequential steps in folate synthesis (dihydropteroate synthase and dihydrofolate reductase) in the organism. P. jirovecii pneumonia classically presents with bilateral ground-glass opacities on imaging and disc-shaped yeast forms seen with silver stain of alveolar exudate. Prophylaxis is continued until the CD4+ count rises above 200/mm³ for at least 3 months on antiretroviral therapy. TMP-SMX also provides prophylaxis against Toxoplasma gondii and some bacterial infections.

• A. Voriconazole targets Aspergillus and other molds, not Pneumocystis jirovecii.
• B. Azithromycin is used for Mycobacterium avium complex prophylaxis, indicated at CD4+ counts below 50/mm³.
• D. Isoniazid with pyridoxine is used for latent tuberculosis treatment, not Pneumocystis prophylaxis, and requires a positive PPD/IGRA first.
• E. Fluconazole is not recommended for routine primary prophylaxis against endemic fungal infections in HIV in current guidelines.

Caseating granulomas in tuberculosis form when CD4+ T helper 1 cells activate macrophages via interferon-gamma, causing the macrophages to try to kill intracellular Mycobacterium tuberculosis using reactive oxygen species and lysosomal enzymes; this process also damages surrounding host tissue, producing the characteristic central caseous necrosis. The IGRA result being positive in this patient confirms sensitization to M. tuberculosis antigens (not cross-reactive with BCG, unlike the skin test), indicating latent infection rather than active disease given the normal chest radiograph and absence of symptoms. A positive IGRA with no symptoms and normal imaging is managed as latent TB. M. tuberculosis has no classic exotoxin; its pathogenesis depends on an intracellular immune-mediated response rather than direct toxin damage.

• A. Mycobacterium tuberculosis does not produce an exotoxin; tissue damage results from the host immune response.
• C. Tuberculosis granuloma formation is a cell-mediated (Th1) response, not an IgE-mediated hypersensitivity reaction.
• D. Neutrophils are not the primary effector cells in granulomatous caseous necrosis; macrophages and T cells predominate.
• E. Granuloma necrosis is driven by activated macrophages, not complement-mediated lysis.

Mycobacterium tuberculosis is an obligate aerobe, and the lung apices have the highest oxygen tension relative to perfusion of any lung region, favoring reactivation and bacillary proliferation there. TNF-alpha, produced by activated macrophages, is essential for maintaining granuloma integrity and containing latent infection; infliximab blocks TNF-alpha, increasing the risk of reactivation of latent tuberculosis, as occurred in this patient. Reactivation classically produces a fibrocaseous cavitary lesion in the apical and posterior segments of the upper lobes due to this relatively high oxygen tension. While apical regions do have a high ventilation-perfusion ratio, this reflects relatively low blood flow (and hence oxygen delivery) relative to ventilation, which is precisely what raises local oxygen tension.

• B. The ventilation-perfusion mismatch in the apex is relevant because it raises oxygen tension, not because it causes turbulence.
• C. Macrophage density is not higher in the apex; oxygen tension is the key determinant of mycobacterial growth there.
• D. Lymphatic drainage differences are not the established explanation for apical localization of reactivation tuberculosis.
• E. Mucociliary clearance is not regionally reduced in the upper lobes; this is not the mechanism for apical TB reactivation.

Histoplasma capsulatum is found in soil contaminated with bird or bat droppings in the Ohio and Mississippi River valleys, and infection follows inhalation of microconidia. On histology within infected macrophages, it appears as small, oval, intracellular budding yeasts measuring 2-4 micrometers in diameter, often described as resembling a 'dot' inside macrophages on silver or Giemsa stains. The acute pulmonary presentation with hilar lymphadenopathy after cave exploration (a classic exposure history) supports this diagnosis. Treatment of moderate to severe disease is with amphotericin B, followed by itraconazole.

• A. Broad-based budding yeasts are characteristic of Blastomyces dermatitidis, found in the Midwest and Southeast but not specifically linked to cave/bat exposure.
• B. Septate hyphae with acute-angle branching describe Aspergillus, typically causing invasive disease in immunocompromised hosts.
• D. Spherules with endospores are characteristic of Coccidioides immitis, associated with Southwest US exposure, not this presentation.
• E. Encapsulated yeast with narrow-based budding describes Cryptococcus neoformans, associated with bird droppings (not bat guano caves) and more classically causing meningitis.

Coccidioides immitis is endemic to the deserts of the Southwestern United States, including California's San Joaquin Valley (hence the nickname 'Valley fever'), and causes an acute pulmonary illness that may be accompanied by erythema nodosum and arthralgias ('desert rheumatism'). In tissue, the organism forms large, thick-walled spherules filled with endospores, which is diagnostic on histology. Infection occurs after inhalation of arthroconidia from disturbed soil, as occurs with construction or excavation work. Most cases are self-limited in immunocompetent hosts, but disseminated disease can occur in immunosuppressed patients or certain ethnic groups.

• B. Paracoccidioides brasiliensis causes a similar spherule-forming disease but is endemic to Latin America, not the Southwestern United States.
• C. Cryptococcus neoformans produces narrow-based budding encapsulated yeast, not spherules, and is not linked to this geographic exposure.
• D. Histoplasma capsulatum produces small intracellular budding yeasts, not spherules, and is linked to river valley bird/bat droppings exposure, not Southwest desert soil.
• E. Blastomyces dermatitidis produces broad-based budding yeasts, not spherules with endospores.

This patient's presentation is classic for allergic bronchopulmonary aspergillosis (ABPA), which occurs in patients with asthma or cystic fibrosis and results from a combined type I (IgE-mediated, immediate hypersensitivity) and type III (IgG-mediated immune complex) hypersensitivity response to Aspergillus fumigatus colonizing the airways, without actual invasion of lung tissue by the fungus. Elevated total IgE, positive Aspergillus-specific IgE and IgG precipitins, and central bronchiectasis with mucoid impaction support this diagnosis. This contrasts with invasive aspergillosis, which involves angioinvasive septate hyphae with acute-angle branching in immunocompromised hosts, and aspergilloma, which is a fungus ball colonizing a pre-existing cavity (such as from prior tuberculosis) without hypersensitivity or invasion.

• A. This describes invasive aspergillosis, which occurs in immunocompromised patients, not the hypersensitivity-driven disease in this asthmatic patient.
• B. This describes an aspergilloma, which would not produce this degree of hypersensitivity (elevated IgE, precipitins) or bronchospasm.
• C. While type III immune complex mechanisms do contribute to ABPA, isolated IgG-complement deposition does not account for the elevated IgE and immediate skin reactivity seen here.
• E. ABPA is primarily type I and III hypersensitivity, not a type IV granulomatous reaction.