Thiopentone sodium is contraindicated in patients with variegate porphyria because it:
- A Inhibits delta-aminolevulinic acid (ALA) synthase permanently
- B Competitively inhibits ferrochelatase preventing heme synthesis
- C Induces ALA synthase and precipitates an acute porphyric crisis ✓
- D Displaces heme from albumin increasing free heme toxicity
Correct answer: C. Induces ALA synthase and precipitates an acute porphyric crisis
Explanation
Barbiturates (thiopentone, methohexital) are potent inducers of hepatic ALA (delta-aminolevulinic acid) synthase, the rate-limiting enzyme in porphyrin synthesis. This precipitates overproduction of porphyrin precursors (ALA and porphobilinogen) in patients with hereditary porphyrias, potentially triggering a life-threatening acute attack with abdominal pain, motor neuropathy, and autonomic instability. Safe alternatives for porphyric patients include propofol, ketamine, etomidate, and nitrous oxide.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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