Pneumocystis jirovecii pneumonia — ABIM Board MCQ

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ModerateInfectious DiseasePneumocystis jirovecii pneumoniaABIM Board

A 42-year-old man with untreated HIV infection has several weeks of progressive dyspnea, nonproductive cough, and fever. His CD4 count is 48 cells/microL. Chest CT shows diffuse bilateral ground-glass opacities, and the serum lactate dehydrogenase level is elevated. Which underlying immune defect most directly predisposes this patient to the suspected pulmonary infection?

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Correct answer: A — Impaired CD4-positive T-cell–mediated immunity

The presentation is characteristic of Pneumocystis jirovecii pneumonia: advanced untreated HIV, a CD4 count far below 200 cells/microL, subacute fever and nonproductive cough, and diffuse ground-glass opacities. Pneumocystis susceptibility principally reflects loss of CD4-positive T-cell–mediated immunity, which normally coordinates macrophage-mediated organism clearance. Defective neutrophil oxidative burst predisposes to catalase-positive organisms, including Aspergillus. Terminal complement deficiency causes recurrent Neisseria infections. Selective IgA deficiency is associated with mucosal respiratory infections and giardiasis, whereas loss of splenic macrophage function increases susceptibility to encapsulated bacteria such as Streptococcus pneumoniae. Therefore, option A is the single best answer.

Reference: NIH Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents With HIV, “Pneumocystis Pneumonia,” updated May 27, 2026. https://clinicalinfo.hiv-stage.od.nih.gov/en/guidelines/hiv-clinical-guidelines-adult-and-adolescent-opportunistic-infections/pneumocystis?view=full