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Pneumocystis jirovecii pneumonia — SCE Acute Medicine MCQ

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HardAcute Respiratory PresentationsPneumocystis jirovecii pneumoniaSCE Acute Medicine

A 46-year-old man is assessed on the acute medical unit. He has progressive exertional dyspnoea, dry cough and weight loss. Oxygen saturation is 93% at rest but falls to 84% after walking and chest radiograph is near-normal. Initial assessment includes relevant observations, bedside tests and urgent blood results. What is the most appropriate investigation?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Arterial blood gas with lactate

Arterial blood gas with lactate is the best answer because ABG helps quantify gas exchange impairment and guides severity-based adjunctive therapy in suspected pneumocystis pneumonia. Plasma glucose and beta-hydroxybutyrate is plausible, but it is less appropriate for the physiology or timing described in this stem. The other distractors either fit a neighbouring presentation, delay definitive treatment, or miss the key immediate risk. Clinical pearl: Exertional desaturation with a relatively clear radiograph is a classic clue.

Reference: BHIVA opportunistic infection guidance; BTS oxygen guideline