**Core Concept**
The ovarian fossa is a small anatomical depression located on the lateral wall of the pelvis, which serves as the site for the ovary to be situated. It is formed by the convergence of several structures, including the internal iliac artery, the ureter, and the obturator nerve.

**Why the Correct Answer is Right**
The ovarian fossa is formed by a combination of the obturator nerve, the ureter, and the internal iliac artery. The obturator nerve is a key contributor, as it runs laterally to the fossa and forms the medial boundary. The ureter also plays a significant role, as it passes medially to the fossa and forms the lateral boundary. The internal iliac artery contributes to the formation of the fossa by providing a medial boundary.

**Why Each Wrong Option is Incorrect**
**Option A:** The iliac artery is not directly involved in forming the ovarian fossa, as the internal iliac artery is the correct answer. The external iliac artery is not a contributor to the ovarian fossa.
**Option B:** The external iliac vein is not directly involved in forming the ovarian fossa, as the internal iliac artery is the correct answer. The external iliac vein does not contribute to the formation of the fossa.
**Option C:** The obturator internus muscle is not directly involved in forming the ovarian fossa, as the obturator nerve is the correct contributor. The obturator internus muscle does not form the fossa.
**Option D:** The hypogastric nerve is not directly involved in forming the ovarian fossa, as the obturator nerve is the correct contributor. The hypogastric nerve does not form the fossa.

**Clinical Pearl / High-Yield Fact**
The ovarian fossa is a critical anatomical landmark for gynecological surgeries, as it provides a safe location for the ovary to be positioned. Familiarity with the boundaries of the ovarian fossa is essential for surgeons to avoid damaging surrounding structures.

**Correct Answer: D. The hypogastric nerve.**