**Core Concept**
The fallopian tubes have different anatomical regions, each with distinct characteristics and clinical significance. The isthmus is the narrow, muscular portion of the fallopian tube, which is closest to the uterus and has the thickest muscular layer.

**Why the Correct Answer is Right**
Sterilization, typically performed via tubal ligation, is most commonly done at the isthmus due to its accessibility and anatomical characteristics. The isthmus has a thicker muscular layer, making it easier to ligate and cut without causing significant damage to surrounding tissues. Additionally, the isthmus is the narrowest portion of the fallopian tube, making it a more effective site for preventing fertilization.

**Why Each Wrong Option is Incorrect**
**Option A:** The ampulla is the widest portion of the fallopian tube, where fertilization typically occurs. Performing sterilization here may not be as effective due to the larger diameter of the tube.
**Option B:** The infundibulum is the distal portion of the fallopian tube, which has fimbriae that help pick up the oocyte from the ovary. This region is less accessible for sterilization due to its delicate nature and proximity to the ovary.
**Option D:** The cornua refers to the portion of the fallopian tube that projects into the uterine cavity. While it is technically possible to perform sterilization here, it is not the most common site due to its complex anatomy and potential for complications.

**Clinical Pearl / High-Yield Fact**
When performing tubal ligation, it's essential to target the isthmus due to its narrow diameter and thicker muscular layer, making it the most effective site for preventing fertilization.

✓ Correct Answer: C. Isthmus