**Core Concept**
Inhibin is a glycoprotein hormone that plays a crucial role in regulating the reproductive axis, particularly in the feedback control of follicle-stimulating hormone (FSH) secretion. It is secreted by the granulosa cells of the ovaries in females and the Sertoli cells of the testes in males.

**Why the Correct Answer is Right**
Inhibin acts by binding to its specific receptors on the surface of pituitary gonadotrophs, which ultimately leads to a decrease in the secretion of follicle-stimulating hormone (FSH). This is achieved through the inhibition of the transcription and translation of genes involved in FSH production. Inhibin binding also results in the activation of protein kinase A (PKA), which further contributes to the suppression of FSH secretion. As a result, inhibin exerts a negative feedback effect on the hypothalamic-pituitary-gonadal axis, thereby regulating the reproductive cycle.

**Why Each Wrong Option is Incorrect**
**Option B:** Luteinizing hormone (LH) is not directly inhibited by inhibin. Instead, the inhibitory effect of inhibin is specific to FSH, as it targets the FSH beta-subunit gene promoter.

**Option C:** Estrogen is not directly inhibited by inhibin. In fact, inhibin and estrogen have a complex interplay in regulating the reproductive axis, but inhibin does not directly suppress estrogen secretion.

**Option D:** Prolactin is not inhibited by inhibin. The regulation of prolactin secretion is primarily governed by the hypothalamic-pituitary axis, with dopamine being the primary inhibitory neurotransmitter.

**Clinical Pearl / High-Yield Fact**
Inhibin levels are used as a marker for ovarian function and are often elevated in conditions of ovarian failure or insufficiency.

**✓ Correct Answer: A. FSH**