Allopurinol was stopped promptly in a patient who developed toxic epidermal necrolysis, yet the skin lesions continue to extend for several days afterwards. The most likely explanation is:
- A Antibodies against allopurinol persist in the circulation for weeks after clearance of the parent drug
- B Oxypurinol, its active metabolite, has a long half-life that is further prolonged in renal impairment ✓
- C T-cell clones primed by the drug remain activated independently of ongoing antigen exposure
- D Rebound uric acid elevation directly worsens keratinocyte apoptosis
Correct answer: B. Oxypurinol, its active metabolite, has a long half-life that is further prolonged in renal impairment
Explanation
Allopurinol is rapidly converted to oxypurinol, which is almost entirely renally eliminated and has a half-life of roughly 18 to 30 hours, considerably longer in patients with reduced renal function. Continued antigen exposure through this lingering metabolite explains disease progression despite stopping the parent drug, and it is why renal impairment and thiazide use increase risk. Option C describes a theoretical mechanism but is not the accepted explanation examined in standard texts.
Reference: Rook's Textbook of Dermatology, 9th ed.
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