**Core Concept**
Erectile dysfunction (ED) is a common side effect of certain antihypertensive medications, particularly those that affect nitric oxide (NO) levels or the relaxation of smooth muscle cells in the corpus cavernosum. The pathophysiology of ED involves the impairment of NO-mediated vascular smooth muscle relaxation, leading to decreased blood flow to the penis.

**Why the Correct Answer is Right**
Diuretics, such as thiazides and loop diuretics, are known to cause ED by reducing NO levels and increasing oxidative stress, which impairs the relaxation of smooth muscle cells in the corpus cavernosum. This leads to decreased blood flow to the penis, resulting in erectile dysfunction. Additionally, diuretics can also cause dehydration, which further exacerbates ED.

**Why Each Wrong Option is Incorrect**
**Option A:** Beta-blockers, such as propranolol, can cause ED, but it is not as common as with diuretics. Beta-blockers primarily affect the heart rate and contractility, rather than the NO-mediated relaxation of smooth muscle cells.
**Option B:** ACE inhibitors, such as lisinopril, are actually beneficial for ED, as they increase NO levels and improve endothelial function. They are often used off-label to treat ED.
**Option C:** Calcium channel blockers, such as verapamil, can cause ED, but it is not as common as with diuretics. Calcium channel blockers primarily affect vascular smooth muscle tone, rather than NO levels.

**Clinical Pearl / High-Yield Fact**
Diuretics are a common cause of ED, and patients on diuretics should be counseled about the potential side effects and advised to stay hydrated to minimize the risk of ED.

**Correct Answer: D. Diuretics**