**Core Concept**
A femoral hernia occurs when tissue, such as a part of the intestine, protrudes through the femoral canal, a narrow passage in the groin area. The inguinal canal, located above the femoral canal, is a separate structure that allows the spermatic cord in males and the round ligament in females to pass through.

**Why the Correct Answer is Right**
In a femoral hernia, the hernia sac protrudes through the femoral canal, which is located medial to the femoral vein. The femoral vein is a major vein that carries deoxygenated blood from the lower extremities back to the heart. The hernia sac, containing a loop of small intestine, will be located immediately medial to the femoral vein, as it protrudes through the femoral canal.

**Why Each Wrong Option is Incorrect**
**Option A:** The inguinal canal is actually located above the femoral canal, not medial to the hernia sac in a femoral hernia.
**Option B:** The femoral artery is located lateral to the femoral vein, not medial to the hernia sac.
**Option C:** The pectineal ligament is a fibrous structure that supports the inguinal ligament, but it is not located medial to the hernia sac in a femoral hernia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that femoral hernias are more common in women than men and are often associated with a higher risk of incarceration and strangulation, which can lead to bowel ischemia and necrosis.

**Correct Answer: D. Femoral vein.