**Core Concept**
Ectopic pregnancy is a life-threatening condition where the fertilized ovum implants outside the uterine cavity, most commonly in the fallopian tube. This condition is a leading cause of maternal mortality in the first trimester.

**Why the Correct Answer is Right**
The most common site of an ectopic pregnancy is the ampulla of the fallopian tube, which is the widest part of the tube and has the most favorable environment for implantation of the fertilized ovum. The ampulla is a muscular and thin-walled region, allowing the fertilized ovum to implant and develop outside the uterus. The fertilized ovum is likely to implant in the ampulla due to its optimal environment, which includes a rich blood supply and a suitable pH.

**Why Each Wrong Option is Incorrect**
**Option A:** The isthmus of the fallopian tube is narrower and has a thicker muscular wall compared to the ampulla, making it less likely for an ectopic pregnancy to occur here.
**Option B:** The ovary is not a common site for ectopic pregnancy, as it is not a part of the fallopian tube where the fertilized ovum usually implants.
**Option C:** The interstitial portion of the fallopian tube is the part that passes through the uterine wall, but it is not the most common site for ectopic pregnancy due to its smaller diameter.

**Clinical Pearl / High-Yield Fact**
The risk of ectopic pregnancy increases with the presence of pelvic inflammatory disease (PID), previous ectopic pregnancy, and assisted reproductive technology (ART) procedures.

**Correct Answer:** C. Interstitial portion of the fallopian tube