**Core Concept**
Clomiphene citrate is a selective estrogen receptor modulator (SERM) used in the treatment of anovulatory infertility. It acts by inducing a positive feedback on the hypothalamic-pituitary-ovarian axis, stimulating the release of gonadotropins (LH and FSH), and subsequently promoting follicular growth and ovulation.

**Why the Correct Answer is Right**
Clomiphene citrate binds to estrogen receptors in the hypothalamus, leading to a decrease in the production of gonadotropin-releasing hormone (GnRH). This decrease triggers a negative feedback loop, which is overridden by the clomiphene-induced upregulation of GnRH receptors. As a result, the pituitary gland releases more LH and FSH, stimulating the ovaries to produce multiple follicles and ovulate. Clomiphene's dual action as an estrogen receptor antagonist and agonist allows it to selectively target the hypothalamic-pituitary-ovarian axis, making it an effective treatment for anovulatory infertility.

**Why Each Wrong Option is Incorrect**
**Option A:** Clomiphene citrate is not primarily used for polycystic ovary syndrome (PCOS) management. Although it may be used off-label in some cases, first-line treatment for PCOS typically involves lifestyle modifications and medications like metformin or spironolactone.
**Option B:** Letrozole is a different medication that also induces ovulation by inhibiting estrogen production, but it is not the primary choice for initial treatment.
**Option C:** Human menopausal gonadotropin (hMG) is a gonadotropin used in ovulation induction and assisted reproductive technologies (ART), but it is not the first-line treatment for anovulatory infertility.

**Clinical Pearl / High-Yield Fact**
When using clomiphene citrate, it's essential to remember that the first cycle often results in ovulation, but the subsequent cycles may require dose adjustments or the addition of other medications to optimize the response.

**Correct Answer: D. Clomiphene citrate.**