American Board of Internal Medicine (ABIM) Certification Practice Exam

In which patient population is febuxostat contraindicated?

1. Patients allergic to purines

2. Patients taking azathioprine

3. Patients with severe liver dysfunction

4. Patients on warfarin therapy

The correct answer is: Patients taking azathioprine

Febuxostat is a xanthine oxidase inhibitor used primarily to manage hyperuricemia in patients with gout. The correct situation in which febuxostat is contraindicated is in patients who are taking azathioprine. Azathioprine is a medication that is metabolized by the enzyme xanthine oxidase, and the use of febuxostat can significantly increase the plasma levels of azathioprine, leading to potential toxicity. This risk arises because febuxostat inhibits the breakdown of thiopurines, including azathioprine, which can result in adverse effects such as severe bone marrow suppression. In other scenarios, such as patients allergic to purines, febuxostat would not be contraindicated purely based on an allergy to purines because febuxostat does not contain purines itself. For patients with severe liver dysfunction, while caution is warranted and close monitoring is necessary, febuxostat can sometimes be used under careful medical supervision; however, renal adjustments are more applicable. Additionally, patients on warfarin therapy do not face a contraindication with febuxostat, though interaction monitoring is advisable as with any medication. Thus, the association of elevated azathioprine levels and the risk of toxicity directly informs the contraind