**Core Concept**
Alprostadil is a synthetic form of prostaglandin E1 (PGE1), a potent vasodilator that plays a crucial role in maintaining vascular tone and blood flow. Its therapeutic effects are primarily mediated through the activation of specific receptors on vascular smooth muscle cells, leading to relaxation and vasodilation.

**Why the Correct Answer is Right**
Alprostadil is used to treat erectile dysfunction (ED) by inducing vasodilation of the penile arteries, thereby increasing blood flow to the penis and facilitating an erection. This is achieved through the activation of PGE1 receptors on the smooth muscle cells of the corpus cavernosum, leading to relaxation and increased blood flow. Additionally, alprostadil has been used to treat pulmonary hypertension, where it helps to reduce pulmonary vascular resistance and improve right ventricular function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as alprostadil is not primarily used to treat hypertension, although it may have some effect on blood pressure due to its vasodilatory properties.
**Option B:** This option is incorrect as alprostadil is not used to treat cardiac arrhythmias, although it may have some effect on myocardial contractility.
**Option C:** This option is incorrect as alprostadil is not used to treat asthma, although it may have some effect on airway smooth muscle tone.

**Clinical Pearl / High-Yield Fact**
Alprostadil can cause local pain, erythema, and priapism (prolonged erection) as side effects, particularly when administered intracavernously. To minimize these risks, it is essential to use the correct dose and administration technique.

**Correct Answer:** D.