Pneumocystis jirovecii pneumonia — DTM&H MCQ

Correct answer: A — Pneumocystis jirovecii pneumonia

The correct answer is A, Pneumocystis jirovecii pneumonia. This is the classic presentation: a man with untreated HIV and a CD4 count well below 200 cells/microlitre developing subacute dry cough, progressive breathlessness, bilateral interstitial (rather than lobar) infiltrates, and hypoxaemia on air. BHIVA guidance identifies reduced CD4 count as the key risk marker, and most PCP occurs in patients with CD4 counts under 200 cells/mm3, with severity assessed by PaO2 or A-a gradient. The combination of insidious onset, bilateral interstitial change and significant hypoxaemia (PaO2 8.5 kPa) at this CD4 level is far more consistent with PCP than with any bacterial or other opportunistic process, and this severity threshold also determines whether adjunctive corticosteroids are required alongside co-trimoxazole. Why the other options are wrong: E. Paragonimiasis: causes a chronic cough with haemoptysis and eosinophilia related to eating undercooked freshwater crustaceans, typically producing pleural effusion or nodular/cavitary change, not the diffuse bilateral interstitial pattern with acute hypoxaemia seen here. D. Pulmonary cryptococcosis: usually presents with nodular or mass-like lesions, often with concurrent meningitis, and is a less common and typically less acutely hypoxaemic presentation than PCP at this CD4 level. C. Bacterial lobar pneumonia: presents acutely with productive cough, fever and focal lobar consolidation on chest radiograph, not the bilateral interstitial infiltrate and subacute course described. B. Pulmonary schistosomiasis: results from egg embolisation causing granulomatous pulmonary hypertension, presenting with dyspnoea and signs of right heart strain over a much longer timescale, not an acute interstitial pneumonitis. Key point: bilateral interstitial infiltrates with subacute dyspnoea and hypoxaemia in a patient with CD4 count under 200 cells/microlitre is PCP until proven otherwise, and the PaO2 value also dictates the need for adjunctive corticosteroids.

Reference: British HIV Association (BHIVA) Guidelines on the Management of Opportunistic Infection in HIV-Seropositive Individuals 2024, Pulmonary section: https://bhiva.org/wp-content/uploads/2024/10/OI-guidelines-Pulmonary.pdf