A 22-year-old man started on a new antiepileptic 6 weeks ago reports difficulty recalling words during conversation and has lost 5 kg. He has also passed two renal calculi. Urine pH is persistently above 7.5. Which drug best fits this profile?
- A Zonisamide
- B Levetiracetam
- C Topiramate ✓
- D Oxcarbazepine
Correct answer: C. Topiramate
Explanation
Topiramate weakly inhibits carbonic anhydrase, producing renal bicarbonate loss, alkaline urine, calcium phosphate stones, and metabolic acidosis. Its well recognised cognitive effects include psychomotor slowing and word-finding difficulty, and it suppresses appetite causing weight loss. Levetiracetam does not cause stones or weight loss. Oxcarbazepine causes hyponatraemia rather than lithiasis. Zonisamide also inhibits carbonic anhydrase but weight loss without prominent word-finding difficulty points to topiramate as the classic exam answer.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.
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