A nurse is caring for a client who is taking allopurinol to treat gout and has a new prescription for azathioprine to treat ulcerative colitis. For which of the following reasons should the nurse clarify these prescriptions with the provider?
A. Allopurinol delays the conversion of azathioprine and can lead to toxicity.
B. Azathioprine increases the effectiveness of allopurinol.
C. Allopurinol increases the metabolism of azathioprine and can require an increased dosage.
D. Azathioprine decreases the effectiveness of allopurinol.