MTX Pneumonitis — SCE Dermatology MCQ

Correct answer: B — Methotrexate-induced pneumonitis — a potentially fatal hypersensitivity reaction requiring immediate Methotrexate cessation

The correct answer is B, Methotrexate-induced pneumonitis, a potentially fatal hypersensitivity reaction requiring immediate Methotrexate cessation. Methotrexate causes an idiosyncratic, non-dose-dependent hypersensitivity pneumonitis that classically presents with gradually progressive dyspnoea, dry cough and bilateral interstitial infiltrates on CXR, exactly as described in this stem. Because it can progress rapidly to respiratory failure and death, it must be actively excluded in any patient on Methotrexate presenting with new respiratory symptoms, and treatment is immediate and permanent drug withdrawal rather than dose reduction or watchful waiting. UK monitoring guidance explicitly instructs that patients on Methotrexate with unexplained dyspnoea or non-productive cough be referred urgently, reflecting the seriousness of this diagnosis. Investigation typically includes HRCT and bronchoalveolar lavage to exclude infection (particularly PJP) before confirming a drug-induced aetiology, but Methotrexate is stopped empirically while this is done given the risk of delay. Why the other options are wrong: C. Pulmonary embolism: PE typically causes acute, not gradually progressive over weeks, breathlessness, and produces a normal or focal CXR rather than bilateral interstitial infiltrates. A. Chest infection only: infection remains a differential to exclude via BAL, but it does not account for the specific drug exposure and characteristic bilateral interstitial pattern, and treating as infection alone risks missing a fatal drug reaction. E. Heart failure only: heart failure causes bilateral shadowing but with cardiomegaly, upper lobe diversion and Kerley B lines rather than a purely interstitial pattern, and there is no clinical indication of cardiac disease here. D. Asthma exacerbation: asthma causes episodic wheeze with a normal CXR, not fixed interstitial infiltrates developing over weeks. Key point: any patient on Methotrexate with progressive dyspnoea and bilateral interstitial CXR changes must have the drug stopped immediately pending exclusion of hypersensitivity pneumonitis, as rechallenge or delay can be fatal.

Reference: NICE BNF, Methotrexate (oral), Side-effects and monitoring: interstitial pneumonitis; NHS methotrexate shared care guidance advising immediate specialist referral for unexplained dyspnoea or cough. https://bnf.nice.org.uk/drugs/methotrexate/