**Question:** Mullerian inhibiting substance (MIS) is produced by

A. Gonads (testes or ovaries)
B. Brain
C. Placenta
D. Circulating blood

**Correct Answer:** C. Placenta

**Core Concept:**
Mullerian inhibiting substance (MIS) is a glycoprotein hormone involved in the development of the female reproductive tract during fetal life. It acts as a repressor of Mullerian duct development, which results in the absence or hypoplasia of the fallopian tubes, uterus, and upper part of the vagina.

**Why the Correct Answer is Right:**
Mullerian inhibiting substance is produced by the placenta, specifically by the trophoblast cells. This is because the MIS plays a crucial role in female reproductive tract development, which occurs during pregnancy. The placenta, being an essential organ in pregnancy, is responsible for producing and secreting MIS, which then reaches the embryonic female developing in the uterus. This ensures that the Mullerian ducts are suppressed, leading to the formation of the female reproductive tract as observed in adults.

**Why Each Wrong Option is Incorrect:**
A) Testes and ovaries are not the primary source of Mullerian inhibiting substance; they are the primary target of MIS action. The MIS produced by placenta is responsible for suppressing Mullerian duct development in the female fetus.
B) The brain does not produce MIS; it is a product of the placenta during pregnancy. Although the hypothalamus does produce gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH), these hormones are not responsible for Mullerian duct suppression.
C) Placenta is the correct answer, as it is the primary source of Mullerian inhibiting substance involved in female reproductive tract development during pregnancy.
D) Circulating blood does not produce MIS; rather, it is produced by the placenta. The MIS travels through the bloodstream to reach the developing female fetus, where it suppresses Mullerian duct development.

**Clinical Pearl:**
Mullerian inhibiting substance plays a crucial role in the development of the female reproductive tract during pregnancy. This is essential for the formation of the female reproductive organs and the absence or hypoplasia of these organs in female fetuses is a result of the action of Mullerian inhibiting substance produced by the placenta. The primary function of MIS is to suppress Mullerian duct development, ensuring the female reproductive tract develops into the unique anatomy observed in females.