A nurse in a provider's clinic is caring for a client who reports erectile dysfunction and requests a prescription for a phosphodiesterase (PDE) inhibitor, sildenafil. Which medication currently prescribed for the client is contraindication to taking sildenafil?

A. Isosorbide mononitrate: This medication is a nitrate used for angina. Concurrent use with sildenafil can cause a significant drop in blood pressure due to additive vasodilatory effects, which may lead to life-threatening hypotension or cardiovascular collapse. B. Acetaminophen: Acetaminophen is a non-opioid analgesic and antipyretic that has no interaction with sildenafil. It does not affect blood pressure or vascular tone and is not a contraindication for PDE inhibitor use. C. Aspirin: Aspirin is an antiplatelet agent used for cardiovascular protection. It does not interact with sildenafil in a way that would make the combination unsafe, although caution is advised if there is a bleeding risk. D. Prednisone: Prednisone is a corticosteroid used for inflammation and autoimmune conditions. It does not have pharmacologic interactions with sildenafil and is not contraindicated for concurrent use.