A 40-year-old man is found unconscious and has been lying on his left side for 14 hours. After recovery, he notices inability to dorsiflex his left foot (foot drop). On examination, there is reduced sensation over the dorsum of the foot.
Which nerve is most likely injured?
Decoding the Stem
Explanation
D. Common peroneal nerve: ✅ Correct. It is the most vulnerable nerve at the fibular neck. Injury leads to loss of dorsiflexion (foot drop), loss of eversion, and sensory loss over the dorsum of the foot.
A. Sciatic nerve: Incorrect. Sciatic injury involves hamstrings and all muscles below the knee; deficit would be much more extensive.
B. Tibial nerve: Incorrect. Controls plantarflexion; injury causes loss of sensation on the sole and inability to stand on tiptoe.
C. Saphenous nerve: Incorrect. A pure sensory branch of the femoral nerve; injury would not cause motor deficit (foot drop).
E. Femoral nerve: Incorrect. Affects the quadriceps (knee extension) and causes sensory loss on the anterior thigh/medial leg.
🧠Peripheral Nerve Quick Ref
| Nerve | Motor Deficit | Sensory Deficit |
|---|---|---|
| Common Peroneal | Foot Drop (Dorsiflexion) | Dorsum of foot |
| Tibial | Plantarflexion loss | Sole of foot |
| Femoral | Knee extension loss | Anterior Thigh |
| Obturator | Adduction loss | Medial Thigh |
Integrated Clinical Questions
1. Which specific muscle is responsible for dorsiflexion of the foot?
2. What are the two terminal branches of the common peroneal nerve?
3. What is the classic gait abnormality seen in foot drop?
4. Beside trauma, what is a common medical cause of common peroneal palsy?
5. What is the "Saturday Night Palsy" equivalent in the lower limb?
⚡ Exam Pearls
- Foot drop = Peroneal nerve until proven otherwise.
- Fibular neck: Always check for a fracture or external pressure at this site in your stem.
- Sensory vs Motor: Deep peroneal = 1st web space; Superficial = Dorsum; CPN = Both.
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