**Core Concept:** Spinal nerves are part of the peripheral nervous system, which consist of nerves originating from the spinal cord. These nerves play a crucial role in transmitting sensory and motor information between the central nervous system and the rest of the body. In humans, the umbilicus (belly button) is a point of convergence for several organs and tissues, including the skin.

**Why the Correct Answer is Right:** The skin at the level of umbilicus is supplied by the T12 spinal nerve. This is because the umbilicus is located in the midline of the abdominal wall and is considered as a point of convergence for the intercostal nerves. The T12 nerve is one of the intercostal nerves that innervates the skin in this area, providing it with sensory and motor functions.

**Why Each Wrong Option is Incorrect:**

A. The skin at the umbilicus is supplied by the T11 spinal nerve. This is incorrect because T12 is the correct spinal level for the umbilicus, not T11.

B. The skin at the umbilicus is supplied by the lumbar plexus. This is incorrect as the lumbar plexus is a network of nerves that derive from the lumbar spinal nerves, not directly innervate the skin. The umbilicus receives its supply from the intercostal nerves, which are derived from the spinal nerves.

C. The skin at the umbilicus is supplied by the sacral plexus. This is incorrect because the sacral plexus is responsible for supplying the lower limbs and genitalia, not the abdominal wall. The umbilicus receives its supply from the intercostal nerves, derived from the spinal nerves.

D. The skin at the umbilicus is supplied by the costal nerves. This is incorrect because the costal nerves are part of the intercostal nerves, which supply the skin at the umbilicus. The spinal nerve responsible for the umbilicus is T12, not costal nerves.

**Clinical Pearl:** The correct understanding of the spinal levels supplying the umbilicus is essential for both clinical practice and the assessment of patients with neurological deficits or pain in the abdominal wall. This knowledge is also useful for reconstructive surgery, where the knowledge of skin innervation is crucial for successful surgical interventions.

**Correct Answer:** T12 (Correct spinal nerve level for the umbilicus)

**Why the Correct Answer is Right:** The skin at the umbilicus is supplied by the T12 spinal nerve, which is a part of the intercostal nerves. The intercostal nerves arise from the T6 to T12 spinal nerves and supply the skin and muscles of the thoracic wall. T12 is the spinal level for the skin innervation of the umbilicus.

**Why Each Wrong Option is Incorrect:**

A. T6: The correct spinal level is T12, not T6. T6 is responsible for the innervation of the skin in the lateral chest wall, not the umbilicus.

B. T11 and T11: The skin innervation for the umbilicus comes from T12, not