Foot drop is caused by damage to:

Correct Answer: D. Common peroneal nerve

Foot drop is the inability to dorsiflex the foot at the ankle, resulting in a characteristic "steppage gait" where the patient lifts the knee higher to clear the drooping foot. This motor deficit occurs due to paralysis of the tibialis anterior muscle, which is the primary dorsiflexor of the foot. The tibialis anterior is innervated by the deep peroneal nerve (also called deep fibular nerve), which is a terminal branch of the common peroneal nerve. The common peroneal nerve arises from the sciatic nerve (L4–S2) and descends along the lateral aspect of the knee, where it is vulnerable to compression injury (e.g., from prolonged squatting, tight casts, or external pressure). Damage at any point along the common peroneal nerve—whether at the knee, fibular head, or proximal leg—will interrupt motor supply to the tibialis anterior and extensor hallucis longus, causing foot drop. This is one of the most clinically relevant nerve injuries in Indian orthopedic and neurology practice, commonly seen in patients with prolonged immobilization, knee surgery, or habitual leg crossing.

Why the other options are wrong

A. Femoral nerve — The femoral nerve (L2–L4) innervates the quadriceps and hip flexors, not the dorsiflexors of the foot. Femoral nerve injury causes weakness of knee extension and hip flexion, not foot drop. This is a trap for students who confuse proximal lower limb innervation with distal motor deficits. B. Sciatic nerve — While the sciatic nerve is the parent trunk of the common peroneal nerve, isolated sciatic nerve injury typically causes foot drop plus loss of plantar flexion and inversion (tibial nerve involvement). Foot drop alone, without these additional deficits, points to the common peroneal nerve specifically, not the entire sciatic nerve. C. Tibial nerve — The tibial nerve innervates the plantar flexors (gastrocnemius, soleus) and invertors of the foot. Tibial nerve injury causes weakness of plantar flexion and inversion, not dorsiflexion. Foot drop requires loss of dorsiflexion, which is the opposite motor action—a key discriminator NBE tests.

High-Yield Facts

Mnemonics

Foot Drop = Peroneal (Deep Branch) Dorsiflexion → Deep peroneal → Damage causes foot drop. Remember: dorsiflexion is the action lost in foot drop, and the deep peroneal nerve is responsible for this action. Sciatic Nerve Branches: PED Peroneal (common peroneal) → Extensor muscles (dorsiflexors) → Drop foot. Tibial branch → plantar flexors → no foot drop.

NBE Trap

NBE pairs "sciatic nerve" with foot drop to trap students who know the common peroneal nerve is a branch of the sciatic nerve but fail to recognize that foot drop is a specific motor deficit requiring the deep peroneal nerve, not the entire sciatic nerve. The trap tests whether students can distinguish between a parent nerve and its functionally specific terminal branches.

Clinical Pearl

In Indian clinical practice, foot drop from common peroneal nerve compression is frequently encountered in orthopedic wards post-knee surgery or in patients with habitual leg crossing. Recognition of this "steppage gait" at the bedside—where the patient lifts the knee excessively to clear the drooping foot—is a rapid diagnostic clue that should immediately prompt examination of the common peroneal nerve at the fibular head for tenderness or compression.

_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 27 (Peripheral Nerve Disorders); Bailey & Love's Short Practice of Surgery, Ch. 52 (Peripheral Nerve Injuries); Clinically Oriented Anatomy by Moore, Ch. 6 (Lower Limb)_