Which drug is contraindicated for a PAH patient using tadalafil due to potential severe interaction?

In patients with pulmonary arterial hypertension (PAH) who are using tadalafil, nitroglycerin is contraindicated due to the risk of severe hypotension. Tadalafil is a phosphodiesterase-5 (PDE-5) inhibitor that can increase levels of nitric oxide and lead to vasodilation. When nitroglycerin, a nitrate that also promotes vasodilation, is used concurrently with tadalafil, it can potentially cause an excessive drop in blood pressure. Nitrates enhance the effects of PDE-5 inhibitors like tadalafil, which is why this combination is particularly dangerous and contraindicated in clinical practice.

Other medications listed, such as furosemide, simvastatin, and nifedipine, do not have the same potential for severe interactions with tadalafil. Furosemide is a diuretic often used in heart failure and fluid retention; simvastatin is used for cholesterol management; and nifedipine, a calcium channel blocker, can be used to treat hypertension or angina but does not interact with tadalafil to the same dangerous degree as nitroglycerin. Therefore, nitroglycerin stands out as the only drug with a significant contraindication when given with tadalafil in PAH patients.