Methotrexate Pneumonitis — SCE Dermatology MCQ

Correct answer: A — Methotrexate-induced pneumonitis

The correct answer is A, Methotrexate-induced pneumonitis. This woman on weekly low-dose methotrexate has developed subacute progressive dyspnoea, dry cough, low-grade fever and bilateral diffuse interstitial infiltrates with a normal white cell count, the classic pattern of methotrexate pulmonary toxicity, which is an idiosyncratic hypersensitivity reaction and not dose-related. The normal white count argues against a bacterial process and supports a drug-induced inflammatory pneumonitis rather than infection. A subacute course over one to two weeks with systemic upset (fever, malaise) plus interstitial rather than lobar or focal change on chest X-ray is the recognised presentation, requiring immediate methotrexate withdrawal, exclusion of infection, and often corticosteroids. Why the other options are wrong: C. Heart failure: would typically show cardiomegaly, upper lobe diversion or bilateral pleural effusions with a more alveolar pattern, and there is no history of cardiac disease, orthopnoea or oedema. E. Pulmonary embolism: causes acute pleuritic pain and sudden breathlessness rather than a two week progressive course with fever, and does not typically produce bilateral diffuse interstitial infiltrates on plain film. B. COVID-19 pneumonia: can cause bilateral change but there is no exposure or testing history given, and the clinical context of established methotrexate use makes a drug reaction far more likely than an unstated viral illness. D. Community-acquired pneumonia: usually causes focal consolidation with a raised white cell count and acute-phase response, whereas here the white cell count is explicitly normal, arguing against bacterial infection. Key point: In any patient on methotrexate developing subacute dyspnoea, dry cough, fever and bilateral interstitial infiltrates with a normal white cell count, methotrexate pneumonitis must be assumed and the drug stopped immediately pending exclusion of infection.

Reference: Joint Formulary Committee. British National Formulary (BNF): Methotrexate, side-effects and monitoring (pulmonary toxicity/pneumonitis). NICE BNF, current edition. https://bnf.nice.org.uk/drugs/methotrexate/