A 30-year-old woman presents with recurrent abdominal pain, psychosis, and peripheral neuropathy. Her urine turns dark red on standing. No skin photosensitivity. Urine and fecal porphyrins: markedly elevated ALA and PBG in urine. The most likely diagnosis and deficient enzyme are:

• A Porphyria cutanea tarda; uroporphyrinogen decarboxylase
• B Congenital erythropoietic porphyria; uroporphyrinogen III cosynthase
• C Acute intermittent porphyria (AIP); porphobilinogen deaminase (hydroxymethylbilane synthase) ✓
• D Variegate porphyria; protoporphyrinogen oxidase

Explanation

Acute intermittent porphyria (AIP) is caused by deficiency of porphobilinogen (PBG) deaminase (hydroxymethylbilane synthase), the third enzyme in heme synthesis. ALA and PBG accumulate and are excreted in urine — the Watson-Schwartz test (Ehrlich's aldehyde reagent) is positive. The cardinal features are the triad of abdominal pain, neuropsychiatric symptoms, and peripheral neuropathy, WITHOUT cutaneous photosensitivity (distinguishing it from variegate porphyria and hereditary coproporphyria). Precipitants include drugs inducing CYP450 (e.g., barbiturates, sulfonamides), fasting, and stress.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.