**Core Concept**
Subcapsular orchiectomy is a surgical procedure where the tissue beneath the outer layer of the testicle is removed, but the testicle itself remains intact. This procedure is often performed to treat certain conditions, such as hormone-sensitive cancers.

**Why the Correct Answer is Right**
The correct answer is prostate cancer. The procedure is done to reduce the level of testosterone in the body, which can fuel the growth of prostate cancer cells. By removing the Leydig cells, which are responsible for producing testosterone, the body's testosterone levels decrease, thereby slowing down the growth of the cancer. This is a common treatment approach for prostate cancer, especially in patients with advanced disease or those who are unable to undergo more extensive surgery.

**Why Each Wrong Option is Incorrect**
**Option A:** Testicular cancer - This is incorrect because testicular cancer typically requires orchiectomy, which involves removing the entire testicle. Subcapsular orchiectomy is not sufficient to treat testicular cancer.
**Option B:** Seminoma - This is incorrect because seminoma, a type of testicular cancer, is typically treated with orchiectomy, not subcapsular orchiectomy.
**Option C:** Hydrocele - This is incorrect because hydrocele, a condition characterized by an accumulation of fluid in the scrotum, is typically treated with aspiration or surgical repair, not subcapsular orchiectomy.

**Clinical Pearl / High-Yield Fact**
Subcapsular orchiectomy is a relatively minor surgical procedure that can be performed under local anesthesia, making it an attractive option for patients who are not good candidates for more extensive surgery.

**Correct Answer: D. Prostate cancer.**