**Core Concept**
A femoral hernia is a type of hernia that protrudes through the femoral canal, a narrow passageway in the groin area. The femoral canal is bounded by the femoral vein medially, the inguinal ligament (Poupart's ligament) inferiorly, and the lacunar ligament (Gimbernat's ligament) anteriorly. The femoral hernia sac contains abdominal contents, often small intestine, which bulges through the weakened area.

**Why the Correct Answer is Right**
The lacunar ligament, also known as Gimbernat's ligament, is a fibrous structure that forms the anterior boundary of the femoral canal. It is located immediately medial to the femoral hernia sac, which protrudes through the femoral canal. The lacunar ligament provides a landmark for identifying the location of the femoral hernia sac during physical examination. In this case, the presence of a bulge immediately inferior to the inguinal canal and medial to the lacunar ligament is consistent with a femoral hernia.

**Why Each Wrong Option is Incorrect**
**Option A:** The femoral artery is located lateral to the femoral hernia sac, not medial. It is a major artery that supplies blood to the lower limb.
**Option B:** The femoral nerve is located lateral to the femoral hernia sac, not medial. It is a nerve that provides sensory and motor innervation to the lower limb.
**Option C:** The femoral vein is located lateral to the femoral hernia sac, not medial. It is a major vein that drains blood from the lower limb.

**Clinical Pearl / High-Yield Fact**
When examining a patient with a suspected femoral hernia, it is essential to identify the location of the lacunar ligament, which serves as a landmark for the femoral canal. The lacunar ligament is often palpable as a firm, fibrous structure in the groin area.

✓ Correct Answer: D. Lacunar ligament.