Porphyrias, deficiencies & genodermatoses flashcards
15 high-yield MRCP Part 1 cards from the Intellect Medicos Dermatology course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.
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Acute intermittent porphyria is due to deficiency of ______; porphyria cutanea tarda is due to deficiency of uroporphyrinogen decarboxylase.
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porphobilinogen deaminase
Why? AIP causes neurovisceral attacks without skin signs; PCT causes photosensitive blistering on sun-exposed skin without acute attacks.
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Acute intermittent porphyria is due to deficiency of porphobilinogen deaminase; porphyria cutanea tarda is due to deficiency of ______.
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uroporphyrinogen decarboxylase
Why? AIP causes neurovisceral attacks without skin signs; PCT causes photosensitive blistering on sun-exposed skin without acute attacks.
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What is the first-line investigation during a suspected acute attack of acute intermittent porphyria?
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Urinary porphobilinogen (raised, along with ALA).
Why? Urine may darken on standing; hyponatraemia (SIADH) is a common accompanying finding.
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What is the specific treatment for an acute attack of acute intermittent porphyria?
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IV haem arginate (haemin), which suppresses ALA synthase — plus stopping precipitants and giving carbohydrate (glucose).
Why? Reducing haem precursor production stops the build-up of toxic ALA and porphobilinogen.
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Name four drugs that can precipitate an attack of acute intermittent porphyria.
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Any four: barbiturates, oral contraceptive pill, halothane, alcohol, sulphonamides, and enzyme inducers such as rifampicin, phenytoin and carbamazepine.
Why? Enzyme inducers and other porphyrinogenic drugs increase ALA synthase activity. Fasting and infection also precipitate attacks.
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How is acute intermittent porphyria inherited?
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Autosomal dominant, with low penetrance (most carriers never have an attack).
Why? Attacks need an additional trigger such as drugs, alcohol, fasting or infection.
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What is the first-line treatment for porphyria cutanea tarda?
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Venesection (low-dose hydroxychloroquine if venesection is unsuitable), plus sun protection and stopping alcohol and oestrogens.
Why? Lowering iron stores restores enzyme activity. Check for hepatitis C and haemochromatosis, which are associated.
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Hereditary haemorrhagic telangiectasia (Osler–Weber–Rendu) is inherited as ______.
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autosomal dominant
Why? Curaçao criteria: recurrent epistaxis, mucocutaneous telangiectasia, visceral AVMs and a first-degree relative with HHT — 3 or more is definite, 2 is possible. Pulmonary AVMs can cause paradoxical embolic stroke.
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Which enzyme is deficient in variegate porphyria?
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Protoporphyrinogen oxidase.
Why? Variegate porphyria combines acute neurovisceral attacks (like AIP) with photosensitive skin blistering (like PCT); acute attacks are treated with IV haem.
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Which antituberculous drug can cause pellagra?
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Isoniazid.
Why? It interferes with the conversion of tryptophan to niacin (vitamin B3), causing photosensitive dermatitis (Casal's necklace), diarrhoea and dementia. Treat with oral nicotinamide.
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Acrodermatitis enteropathica — periorificial and acral dermatitis with alopecia and diarrhoea — is caused by ______ deficiency.
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zinc
Why? The inherited form is autosomal recessive (SLC39A4 zinc transporter); acquired deficiency follows alcoholism, parenteral nutrition or malnutrition. Treatment is oral zinc.
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Mutations in which two genes cause most cases of hereditary haemorrhagic telangiectasia?
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ENG (endoglin; HHT type 1) and ACVRL1 (ALK1; HHT type 2).
Why? Both encode endothelial TGF-beta pathway proteins. HHT1 carries more pulmonary and cerebral AVMs; HHT2 more hepatic AVMs.
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Neurofibromatosis type 1: on which chromosome is the causative gene, and which protein does it encode?
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Chromosome 17 — neurofibromin (a tumour suppressor that inhibits Ras), encoded by the NF1 gene.
Why? Autosomal dominant. Skin signs: 6 or more café-au-lait macules, axillary/inguinal freckling and cutaneous neurofibromas; Lisch nodules on the iris.
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Which two genes cause tuberous sclerosis complex?
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TSC1 (hamartin, chromosome 9) and TSC2 (tuberin, chromosome 16).
Why? Autosomal dominant. Skin signs: ash-leaf macules (best seen with a Wood's lamp), facial angiofibromas, shagreen patch and periungual fibromas.
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Which gene is mutated in Gorlin (naevoid basal cell carcinoma) syndrome?
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PTCH1 (patched — hedgehog signalling pathway).
Why? Autosomal dominant: multiple early-onset BCCs, palmar pits, odontogenic jaw keratocysts and falx calcification. Hedgehog pathway inhibitors (e.g. vismodegib) are used for advanced BCC.