Volatile Agent Metabolism — FRCA Primary MCQ
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Correct answer: B — Halothane
Halothane is correct. Approximately 20% of absorbed halothane undergoes hepatic biotransformation, predominantly through CYP2E1-dependent oxidative metabolism; reductive metabolism becomes more important under hypoxic conditions. This is substantially greater than sevoflurane, of which less than 5% is metabolised to hexafluoroisopropanol and inorganic fluoride. Isoflurane undergoes only about 0.2% metabolism, while desflurane is still more resistant to biotransformation at approximately 0.02%. Nitrous oxide undergoes no clinically significant hepatic metabolism and is predominantly eliminated unchanged through the lungs. Sevoflurane is the most plausible distractor because its fluoride-producing metabolism is clinically memorable, but its metabolised fraction remains much lower than that of halothane.
Reference: National Institute of Diabetes and Digestive and Kidney Diseases. Halogenated Anesthetics. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury, updated 2018. https://pubmed.ncbi.nlm.nih.gov/31644158/