**Core Concept**
The inguinal ligament, also known as Poupart's ligament, is a fibrous band that forms the floor of the inguinal canal. It is a significant anatomical landmark in the lower abdominal and pelvic region. Understanding the structures that pass behind this ligament is crucial for identifying and diagnosing various clinical conditions.

**Why the Correct Answer is Right**
The psoas major muscle and the femoral vein pass behind the inguinal ligament. The psoas major muscle is a long, flat muscle that originates from the lumbar vertebrae and inserts into the femur, playing a key role in hip flexion. It lies deep to the inguinal ligament. The femoral vein, on the other hand, is a major vein that drains the lower limb, and it passes behind the inguinal ligament to join the external iliac vein. This is in contrast to the femoral artery, which passes in front of the inguinal ligament.

**Why Each Wrong Option is Incorrect**
**Option A:** The femoral branch of the genitofemoral nerve and the superficial epigastric artery do not pass behind the inguinal ligament. The femoral branch of the genitofemoral nerve passes in front of the inguinal ligament, while the superficial epigastric artery is a branch of the femoral artery that passes in front of the inguinal ligament as well.

**Option C:** The saphenous vein does not pass behind the inguinal ligament. It passes in front of the ligament, ascending from the foot to the thigh.

**Option D:** Not all of the listed structures pass behind the inguinal ligament.

**Clinical Pearl / High-Yield Fact**
When performing inguinal hernia repairs or identifying the inguinal canal, it is essential to remember that the psoas major muscle and the femoral vein pass behind the inguinal ligament, while the femoral artery and the saphenous vein pass in front of it.

**✓ Correct Answer: B. 3 & 4. Psoas major, Femoral vein.**