Porphyria cutanea tarda — SCE Dermatology MCQ

Correct answer: D — Plasma, urine and faecal porphyrin analysis

The correct answer is D, plasma, urine and faecal porphyrin analysis. This man has the classic picture of porphyria cutanea tarda (PCT): skin fragility, erosions and tense blisters over sun-exposed, trauma-prone dorsal hands, facial hypertrichosis, and known triggers (alcohol excess, hepatitis C) that impair hepatic uroporphyrinogen decarboxylase activity. The dark urine on standing reflects photo-oxidation of excess uroporphyrinogen to coloured porphyrins. Diagnosis is confirmed biochemically, not histologically or immunologically, by demonstrating a characteristic elevated urinary and faecal porphyrin pattern with a positive plasma porphyrin fluorescence scan, which is the recognised first-line laboratory approach for cutaneous porphyrias in the UK. Treatment (venesection, low-dose hydroxychloroquine, alcohol cessation, hepatitis C treatment) follows only once this pattern is confirmed. Why the other options are wrong: A. Patch testing to rubber accelerators: this investigates allergic contact dermatitis, which does not explain hypertrichosis, dark urine or the liver/alcohol/hepatitis C risk factors. C. Skin-prick testing to house-dust mite: assesses IgE-mediated inhalant allergy, irrelevant to a photo-aggravated blistering dermatosis with systemic metabolic triggers. B. Bacterial swab from a blister roof: useful only if secondary infection is suspected, it cannot identify the underlying metabolic cause of fragility and blistering. E. Serum pemphigoid antibody alone: bullous pemphigoid causes tense blisters but not hypertrichosis or dark urine, and antibody testing alone would miss PCT while risking misdiagnosis. Key point: dorsal hand fragility and blistering plus hypertrichosis, dark urine and hepatic risk factors (alcohol, hepatitis C) should trigger plasma, urine and faecal porphyrin analysis to confirm porphyria cutanea tarda.

Reference: British Porphyria Association / Woolf J et al, Best practice guidelines on first-line laboratory testing for porphyria, Annals of Clinical Biochemistry 2017 (BIPNET UK specialist laboratory guidance); https://porphyria.org.uk/testing-screening/