A nurse in a provider's clinic is caring for a client who reports erectile dysfunction and requests a prescription for sildenafil. Which of the following medications currently prescribed for the client is a contraindication to taking sildenafil?

A. Phenytoin, an anticonvulsant, does not have a direct contraindication with sildenafil, although it can lower the blood concentration of sildenafil due to enzyme interactions. B. Prednisone, a corticosteroid, does not have a direct contraindication with sildenafil. C. Metronidazole, an antibiotic, does not contraindicate sildenafil. D. Isosorbide is a nitrate used for angina. When combined with sildenafil, there is a significant risk of severe hypotension, which can be life-threatening. Therefore, the two medications are contraindicated.