**Core Concept**
Müllerian-Inhibiting Substance (MIS), also known as Anti-Müllerian Hormone (AMH), is a protein hormone crucial for the development of the reproductive system in males. It plays a pivotal role in the regression of the Müllerian ducts, which would otherwise develop into female reproductive organs.

**Why the Correct Answer is Right**
The Sertoli cells in the testes are responsible for secreting MIS, which then inhibits the growth of the Müllerian ducts. This process is essential for the development of the male reproductive system. The Sertoli cells, also known as nurse cells, play a vital role in the support and maintenance of spermatogenesis. They produce and secrete various factors that regulate the development of sperm cells. The secretion of MIS by Sertoli cells is a critical aspect of male sexual differentiation.

**Why Each Wrong Option is Incorrect**
**Option B:** Leidig cells, also known as Leydig cells, are responsible for producing testosterone in the testes, not MIS. They play a crucial role in the development of male secondary sexual characteristics.
**Option C:** Supporting cells are not a specific term in the context of testicular development. While Sertoli cells do provide support to developing sperm cells, the term "supporting cells" is too vague and does not accurately describe the cell type responsible for MIS secretion.
**Option D:** Seminiferous tubules are the site where spermatogenesis occurs, but they are not responsible for secreting MIS. The Sertoli cells, which line the seminiferous tubules, are the actual source of MIS.

**Clinical Pearl / High-Yield Fact**
MIS/AMH is also used as a marker of ovarian reserve in females, with lower levels indicating a reduced number of oocytes.

**Correct Answer:** **✓ Correct Answer: A. Sertoli cell**