**Core Concept**
Apalutamide is a non-steroidal androgen receptor inhibitor used in the treatment of prostate cancer. It selectively binds to the androgen receptor, preventing the receptor from binding to androgen hormones such as testosterone and dihydrotestosterone, which are necessary for the growth of prostate cancer cells.

**Why the Correct Answer is Right**
Apalutamide was approved by the FDA for the treatment of non-metastatic castration-resistant prostate cancer, a condition where prostate cancer continues to grow despite hormone therapy. It has been shown to slow the progression of the disease, reducing the risk of metastasis and improving overall survival. The mechanism of action of apalutamide involves the inhibition of the androgen receptor, which is crucial for the growth and proliferation of prostate cancer cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because apalutamide is not primarily used for the treatment of metastatic prostate cancer. While it may be used in some cases, its approval was specifically for non-metastatic castration-resistant prostate cancer.
**Option B:** This option is incorrect because apalutamide is not a chemotherapy agent. It is a targeted therapy that works by inhibiting the androgen receptor, rather than killing cancer cells through cell cycle disruption or DNA damage.
**Option C:** This option is incorrect because apalutamide is not used for the treatment of breast cancer. It is specifically approved for the treatment of prostate cancer.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that apalutamide is not a hormone therapy, but rather a targeted therapy that works by inhibiting the androgen receptor. This is a key distinction from traditional hormone therapies, which aim to reduce androgen levels in the body.

**Correct Answer: A. Treatment of non-metastatic castration-resistant prostate cancer.**