**Core Concept**
The pyramidalis muscle is a small, triangular muscle located in the anterior abdominal wall. It plays a minor role in the stabilization and support of the abdominal wall, and its innervation is crucial for its function.

**Why the Correct Answer is Right**
The pyramidalis muscle is innervated by the third lumbar nerve (L3), which is a branch of the lumbar plexus. The L3 nerve carries fibers from the ventral rami of the spinal nerves, which synapse with the spinal cord to control the muscle's contraction. This innervation allows the pyramidalis muscle to contract and relax in response to neural signals, contributing to the dynamic stabilization of the abdominal wall.

**Why Each Wrong Option is Incorrect**
**Option A:** The ilioinguinal nerve, which is responsible for the innervation of the external oblique muscle and the skin of the inguinal region, is not associated with the pyramidalis muscle.
**Option B:** The subcostal nerve, which is the lowest thoracic nerve (T12), is responsible for the innervation of the diaphragm but not the pyramidalis muscle.
**Option C:** The obturator nerve, which is responsible for the innervation of the adductor muscles of the thigh, is not associated with the pyramidalis muscle.

**Clinical Pearl / High-Yield Fact**
The pyramidalis muscle is a small but distinct muscle of the anterior abdominal wall, and its innervation is crucial for its function. Remember that the pyramidalis muscle is innervated by the third lumbar nerve (L3), which is a key fact for identifying its role in the lumbar plexus.

**Correct Answer:** D.