**Core Concept**
Nilutamide is a type of nonsteroidal antiandrogen (NSAA) used primarily in the treatment of prostate cancer. It works by inhibiting the action of androgens, such as testosterone, which can fuel the growth of prostate cancer cells.

**Why the Correct Answer is Right**
Nilutamide is a competitive antagonist of the androgen receptor, which means it binds to the receptor and prevents androgens like testosterone from binding and activating the receptor. This leads to a decrease in the growth and proliferation of prostate cancer cells. The exact mechanism involves the blockade of androgen receptors by nilutamide, thereby preventing the transcription of genes that promote cell growth and survival.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not a selective estrogen receptor modulator (SERM), which is a different class of drugs that can either stimulate or block estrogen receptors.
**Option B:** Incorrect because it is not a gonadotropin-releasing hormone (GnRH) agonist, which works by reducing the production of testosterone in the body.
**Option C:** Incorrect because it is not a 5-alpha-reductase inhibitor, which blocks the conversion of testosterone to dihydrotestosterone (DHT), a more potent form of testosterone.

**Clinical Pearl / High-Yield Fact**
Nilutamide is often used in conjunction with other treatments, such as surgery or radiation therapy, to manage prostate cancer. It's essential to monitor patients for potential side effects like gynecomastia (breast enlargement) and liver enzyme elevations.

**Correct Answer:** C.