A 58-year-old man presents with a two-month history of fatigue, weight loss, myalgias, and mononeuritis multiplex (foot drop). He develops acute hemoptysis and oliguria. Laboratory findings show creatinine 3.8 mg/dL, hematuria with RBC casts, and hemoglobin 8.1 g/dL. Chest X-ray reveals bilateral patchy infiltrates. Spirometry demonstrates a restrictive pattern (TLC 70% predicted). The pattern marked **B** in the diagram shows elevated DLCO (148% predicted). p-ANCA with MPO specificity is positive at high titer; PR3-ANCA is negative. Renal biopsy confirms pauci-immune necrotizing crescentic glomerulonephritis without granulomas. Which of the following best explains the elevated DLCO finding in pattern **B**?
A. Elevated hemoglobin from acute hemolysis increases oxygen-carrying capacity and CO binding
B. Alveolar blood avidly binds carbon monoxide in diffuse alveolar hemorrhage, increasing measured DLCO despite restrictive physiology
C. Granulomatous inflammation in the lungs creates a larger surface area for gas exchange
D. Increased alveolar-capillary membrane thickness from pulmonary fibrosis enhances gas transfer
Explanation
Why option 1 is correct
The elevated DLCO in pattern B (restrictive pattern with elevated DLCO) is the pathognomonic finding of diffuse alveolar hemorrhage (DAH) in microscopic polyangiitis. In DAH, intra-alveolar blood and hemoglobin act as an additional reservoir for carbon monoxide binding, artificially elevating the measured DLCO despite the restrictive physiology caused by alveolar filling. This is a hallmark feature of MPA-associated pulmonary capillaritis and distinguishes it from other restrictive lung diseases. Harrison's 21e and the Chapel Hill Consensus 2012 emphasize that the combination of restriction with elevated DLCO is the signature spirometric pattern of DAH in ANCA-associated vasculitis.
Why each distractor is wrong
Option 2: Pulmonary fibrosis does cause restriction, but it typically results in reduced DLCO (not elevated), as fibrosis damages the alveolar-capillary interface. This pattern is seen in chronic interstitial lung disease, not acute DAH.
Option 3: While hemoglobin is indeed the vehicle for CO binding, acute hemolysis (which would lower hemoglobin) does not explain elevated DLCO. The elevated DLCO in DAH is due to intra-alveolar blood, not systemic hemoglobin levels. The patient's anemia (Hb 8.1 g/dL) would actually reduce overall CO-carrying capacity.
Option 4: Granulomatous inflammation is a hallmark of granulomatosis with polyangiitis (GPA), not microscopic polyangiitis. MPA is explicitly defined by the absence of granulomas and typically does not affect upper airways. Granulomas would not explain elevated DLCO.
High-YieldNEET PG
Elevated DLCO in a restrictive pattern = diffuse alveolar hemorrhage (DAH); intra-alveolar blood binds CO avidly. MPA causes DAH via pulmonary capillaritis without granulomas; GPA causes granulomatous inflammation and upper airway disease.
Harrison's 21e — ANCA-Associated Vasculitis; Chapel Hill Consensus 2012
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