Which nerve innervates the quadratus lumborum?

• A. Subcostal nerve (T12).
• B. Femoral nerve.
• C. Obturator nerve.
• D. Sciatic nerve.

Answer: (A)

The main idea is that the quadratus lumborum is innervated by the subcostal nerve (T12), with additional input from the first lumbar nerves in many individuals. The subcostal nerve runs along the inferior border of the 12th rib and supplies muscles of the posterior and lateral abdominal wall, including the quadratus lumborum. The other nerves listed are dedicated to different regions: the femoral nerve supplies the anterior thigh, the obturator nerve to the medial thigh, and the sciatic nerve to the posterior thigh and leg. Because only the subcostal nerve corresponds to the posterior abdominal wall muscle in question, it is the correct innervation.