A 26-year-old man comes to the physician following an episode of bleeding while brushing his teeth. Physical examination shows petechiae on the chest and arms. Laboratory studies show:
Hemoglobin 9.0 g/dL
WBCs 85,000/μL
Platelets 10,000/μL
Blasts 30%
Prior to commencement of a chemotherapeutic regimen, in order to decrease this patient’s risk of renal failure, it is most appropriate to administer a drug that inhibits which of the following?