**Question:** Which of the following drugs is associated with priapism?

A. Isosorbide dinitrate
B. Sildenafil
C. Nitroglycerin
D. Papaverine

**Core Concept:** Priapism is a condition characterized by prolonged and persistent erection, typically due to a disruption in the normal regulation of penile erection. It is typically classified as either ischemic (low-flow) or non-ischemic (high-flow) based on the underlying cause.

**Why the Correct Answer is Right:** Nitroglycerin is a vasodilator that works by inhibiting the enzyme guanylate cyclase, leading to increased levels of cyclic guanosine monophosphate (cGMP) and relaxation of smooth muscle cells in blood vessels. This resultant increase in blood flow can cause priapism, particularly in ischemic type.

**Why Each Wrong Option is Incorrect:**

A. Isosorbide dinitrate: Similar to nitroglycerin, isosorbide dinitrate is a nitrate vasodilator. However, it has a slightly different mechanism of action, involving the inhibition of NOS and the subsequent release of nitric oxide. This nitric oxide further activates guanylate cyclase, leading to increased cGMP levels and smooth muscle relaxation in blood vessels. Although priapism can occur with isosorbide dinitrate, it is less common than with nitroglycerin.

B. Sildenafil (Viagra) and tadalafil (Cialis) are phosphodiesterase type 5 (PDE5) inhibitors. They work by inhibiting the enzyme PDE5, which leads to increased levels of cGMP and smooth muscle relaxation in blood vessels, particularly in the corpus cavernosum. Although priapism can occur with PDE5 inhibitors, it is a rare side effect.

C. Papaverine is another phosphodiesterase inhibitor, specifically targeting PDE1, PDE3, and PDE4 enzymes. It works by increasing the levels of cGMP and causing smooth muscle relaxation in blood vessels. Priapism can occur with papaverine use, although it is less common than with nitroglycerin or isosorbide dinitrate.

**Clinical Pearl:** Priapism is a rare but serious side effect of certain medications, particularly nitrates and phosphodiesterase inhibitors. It is essential for clinicians to be aware of this potential complication when prescribing these drugs, particularly in patients with underlying vascular disorders or those taking other medications that may contribute to priapism. Prompt recognition and intervention is crucial to prevent complications and allow for timely management of the underlying cause.