**Question:** Anatomical sphincter of fallopian tubes:

A. internal sphincter
B. isthmus of fallopian tube
C. external sphincter
D. smooth muscle layer

**Correct Answer:** C. external sphincter

**Core Concept:** The fallopian tubes are part of the female reproductive system, connecting the ovaries to the uterus. They play a crucial role in ovulation and fertilization. The fallopian tubes contain smooth muscle layers, which are essential for their functions.

**Why the Correct Answer is Right:** The correct answer, the external sphincter, is a part of the smooth muscle layer in the fallopian tubes. This sphincter controls the opening and closing of the tube, ensuring the proper movement of the ovum and sperm during sexual intercourse and ovulation. The external sphincter helps maintain the integrity of the fallopian tube and prevents unwanted leakage or rupture.

**Why Each Wrong Option is Incorrect:**

A. Internal sphincter: This term is incorrect as there is no internal sphincter in the fallopian tubes. The fallopian tubes have smooth muscle layers, but the internal sphincter is typically associated with the gastrointestinal tract.
B. Isthmus of fallopian tube: The isthmus of the fallopian tube is the transition zone between the proximal and distal parts of the fallopian tube. However, it does not represent a specialized muscle layer or structure controlling the opening and closing of the fallopian tube.
D. Smooth muscle layer: While the fallopian tubes contain smooth muscle layers, the question specifically asks for the sphincter that controls the opening and closing of the fallopian tube. The external sphincter is the correct term for this structure.

**Clinical Pearl:** The external sphincter of the fallopian tubes is an essential component in maintaining the integrity of the tubes and ensuring the proper transport of the ovum and sperm during sexual intercourse and ovulation. Understanding the anatomy and function of this structure is crucial for students studying human reproduction and gynecology.