**Core Concept:** Foot drop is a clinical sign characterized by weakness or atrophy of the muscles involved in flexion of the foot at the ankle joint. It can be caused by various conditions affecting the nerves or muscles involved in foot movement.

**Why the Correct Answer is Right:** Foot drop is primarily caused by injury to the commonest sensory and motor peripheral nerves in the lower limb, namely the **L4-S3 spinal nerves** (L5, S1, S2, and S3). These nerves, collectively known as the anterior tibial nerve, provide motor innervation to the muscles involved in dorsiflexion of the ankle joint and flexion of the toes. When these nerves are damaged, the muscles' function is impaired, leading to foot drop.

**Why Each Wrong Option is Incorrect:**

A. **Injury to the common peroneal nerve (L5, S1)** typically results in weakness or atrophy of the muscles responsible for eversion of the foot, not foot drop.

B. **Injury to the tibial nerve (L4, L5, S1, S2, S3)** may cause other symptoms, such as loss of sensation in the territory of the affected nerve, but not foot drop.

C. **Injury to the sciatic nerve (L4-S3)** would typically cause symptoms affecting the lower leg, knee, and ankle, not specifically foot drop.

D. **Injury to the posterior tibial nerve (L8, S1, S2, and S3)** would typically present with symptoms affecting the lower leg, not foot drop.

**Clinical Pearl:** Foot drop can be caused by various factors, including trauma, compression, or neuropathy. Evaluating the patient's history, neurological examination, and imaging studies can help determine the specific nerve or condition causing the foot drop. Treatment depends on the underlying cause and may include physical therapy, orthotics, nerve decompression surgery, or rehabilitation.