**Core Concept**
Benign prostatic hyperplasia (BPH) is a non-cancerous enlargement of the prostate gland, leading to urinary symptoms. In some cases, medication may be used to shrink the prostate size and alleviate symptoms. The goal is to reduce the prostate volume and improve urine flow.

**Why the Correct Answer is Right**
The correct answer is a 5-alpha-reductase inhibitor, which works by blocking the conversion of testosterone to dihydrotestosterone (DHT), a potent androgen that promotes prostate growth. DHT is the primary hormone responsible for the growth of the prostate gland. By inhibiting DHT production, these medications can lead to a shrinkage of the prostate size. This mechanism is crucial in treating BPH symptoms and improving quality of life for patients.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because alpha-blockers are used to relax smooth muscle in the prostate and bladder neck, improving urine flow, but they do not shrink the prostate size. They are often used in combination with 5-alpha-reductase inhibitors.
* **Option B:** This is incorrect because phosphodiesterase-5 inhibitors are used to treat erectile dysfunction by increasing blood flow to the penis, not to treat BPH.
* **Option C:** This is incorrect because finasteride is a 5-alpha-reductase inhibitor, but it is not the only option, and the question is asking for a more specific answer.

**Clinical Pearl / High-Yield Fact**
When treating BPH, it's essential to note that 5-alpha-reductase inhibitors like finasteride or dutasteride can take several months to show significant effects, and patients may require continued treatment to maintain prostate size reduction.

**Correct Answer:** D.