Opportunistic Infections: The USMLE Step 1 Guide

Answer: A. Pneumocystis jirovecii pneumonia is an AIDS-defining opportunistic infection that occurs almost exclusively when CD4+ T helper cell counts fall below approximately 200 cells/microL, because effective clearance of this organism from alveolar spaces depends on CD4+ T-cell-mediated activation of alveolar macrophages and coordination of the cellular immune response. This explains why HIV, which selectively depletes CD4+ T cells, dramatically increases susceptibility to this infection, producing the classic bilateral, diffuse, perihilar ground-glass infiltrates and cup-shaped organisms seen on silver stain. CD8+ T-cell function, humoral immunity, neutrophil counts, and complement activity are relatively preserved in early-to-moderate HIV infection and are not the primary determinants of susceptibility to this particular opportunistic pathogen.

Answer: B. Rabies is a fatal CNS infection caused by rabies virus (an RNA virus in the genus Lyssavirus). The pathophysiology is distinctive for its retrograde axonal transport: following inoculation through a bite wound, the virus enters nerve terminals of peripheral sensory and motor neurons and travels retrogradely (toward the cell body) along axons to reach the spinal cord and brain. This retrograde axonal transport allows the virus to evade systemic immune detection during the incubation period, explaining the variable latency (typically 1–3 months, but can be years) and the ineffectiveness of post-exposure prophylaxis if initiated after CNS invasion. Once in the brain, the virus replicates predominantly in neurons, particularly in the hippocampus, medulla oblongata, cortex, and brainstem. Viral replication causes cytopathic effects leading to neuronal dysfunction and death. Negri bodies are pathognomonic intracytoplasmic inclusions composed of viral ribonucleoproteins and cellular organelles; they appear as eosinophilic, oval to round structures within the neuronal cytoplasm, especially in hippocampal pyramidal neurons, though they are only present in ~80% of rabies cases. The characteristic clinical manifestations reflect viral involvement of specific brain regions: hydrophobia (throat spasm with swallowing or sight of water) reflects involvement of brainstem nuclei controlling swallowing reflexes, particularly the nucleus ambiguus; aerophobia (fear of air currents) reflects hyperexcitability of respiratory control centers; autonomic instability reflects brainstem and limbic involvement. Encephalitis shows lymphocytic pleocytosis in CSF with normal glucose, reflecting viral infection rather than bacterial infection. The disease progresses relentlessly to coma and death despite supportive care, with survival being extraordinarily rare. Negri bodies are diagnostic when present but are not required for diagnosis; PCR of saliva or CSF is the standard diagnostic method. Option A incorrectly emphasizes microglial infection and secondary infection; option C mistakenly attributes encephalitis to vasculitis; option D invokes immune complex-mediated disease; option E overemphasizes glial-mediated inflammation.

Answer: B. Pneumocystis jirovecii pneumonia (PCP) is an opportunistic infection that occurs when CD4+ T-cell counts fall below 200 cells/µL, indicating severe T-cell-mediated immune depletion. CD4+ T cells are essential for activating macrophages (Th1 response) and initiating antibody responses (Th2 response); their absence impairs both arms of adaptive immunity, allowing Pneumocystis to proliferate unchecked within alveoli. The organism triggers a foamy alveolar macrophage response: alveolar macrophages ingest organisms but cannot kill them effectively without T-cell help; the macrophages become lipid-laden and accumulate within alveolar spaces, creating characteristic "foamy" appearance histologically. This inflammatory response fills alveoli with a mixture of organisms, macrophages, fibrin, and lipid material, appearing as ground glass opacification on imaging. The typical distribution is bilateral and diffuse with perihilar predominance due to preferential airway involvement and peribronchial inflammation. Lactate dehydrogenase (LDH) is markedly elevated due to alveolar inflammation. Option A incorrectly denies the importance of CD4+ counts; option C overemphasizes biofilm formation; options D and E invoke incorrect mechanisms (mast cell degranulation, lactic acid-mediated proliferation).