**Core Concept**
Sertoli cells, also known as nurse cells, play a crucial role in the development and maintenance of spermatogenesis in the male reproductive system. They provide structural support, regulate the blood-testis barrier, and secrete various substances essential for spermatogenesis.

**Why the Correct Answer is Right**
Inhibin is a protein complex secreted by Sertoli cells that inhibits the production of follicle-stimulating hormone (FSH) from the anterior pituitary gland. This feedback mechanism is essential for regulating the spermatogenic cycle and maintaining testicular homeostasis. Inhibin consists of two subunits: alpha (α) and beta (β), with the βB subunit being specific to the testes. Sertoli cells produce inhibin in response to FSH stimulation, creating a negative feedback loop that helps regulate the spermatogenic cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** Testosterone is a steroid hormone produced by Leydig cells, not Sertoli cells. While testosterone is essential for spermatogenesis, it is not secreted by Sertoli cells.
**Option B:** Androgen refers to a class of hormones, including testosterone, that promote male characteristics. While Sertoli cells do respond to androgens, they do not secrete them.
**Option C:** FSH is a hormone produced by the anterior pituitary gland that stimulates Sertoli cells to produce androgen-binding protein and inhibit production of inhibin. It is not secreted by Sertoli cells.

**Clinical Pearl / High-Yield Fact**
Inhibin B is a useful marker for assessing spermatogenesis and testicular function, particularly in the context of male infertility. Low levels of inhibin B are associated with impaired spermatogenesis and may indicate testicular dysfunction.

**✓ Correct Answer: D. Inhibin**