Acute intermittent porphyria (AIP) is caused by deficiency of porphobilinogen deaminase (PBGD, also called hydroxymethylbilane synthase). Attacks are precipitated by drugs, fasting, and hormones. A patient in an acute attack has abdominal pain, hyponatremia (SIADH), and peripheral neuropathy with preserved skin findings. Which laboratory finding is PATHOGNOMONIC during an AIP attack?

• A Elevated serum porphobilinogen (PBG) with normal ALA
• B Elevated urine ALA and PBG with normal urine porphyrins
• C Elevated urine uroporphyrin I with normal PBG and ALA
• D Elevated urine porphobilinogen (PBG) and delta-aminolevulinic acid (ALA) with fresh urine turning red/brown on standing ✓

Explanation

In acute attacks of AIP, the partial block at PBGD causes accumulation of its substrate porphobilinogen (PBG) and its precursor ALA. These are excreted in urine in large quantities. Classic teaching: fresh urine is normal yellow, but on standing in light, PBG spontaneously polymerizes to porphyrins (dark red/brown color) — the urine darkens. This positive Watson-Schwartz test (or Hoesch test for PBG) is pathognomonic. Unlike porphyria cutanea tarda, AIP patients do NOT have photosensitivity because uroporphyrins (photosensitizers) do not accumulate significantly — the block is upstream at PBGD before uroporphyrin formation. Urine PBG and ALA are the key diagnostic markers; they normalize between attacks.

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

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