Okay, let's tackle this question. The patient had Bell's palsy, which is facial nerve palsy. The treatment involved a nerve graft using a cutaneous nerve from the lower limb. Now, she's got an area of numbness on the back of the leg laterally and the lateral foot. The question is asking which nerve was used for the graft.

First, I need to recall the cutaneous nerves of the lower limb that are commonly used for grafts. The sural nerve comes to mind because it's often used in nerve grafting procedures. The sural nerve provides sensation to the lateral foot and the posterior leg. If that's the case, then removing the sural nerve would leave those areas numb.

Let me break it down. The sural nerve is formed by the union of the medial sural cutaneous nerve (a branch of the tibial nerve) and the lateral sural cutaneous nerve (a branch of the common peroneal nerve). It runs down the leg and innervates the lateral aspect of the foot and the posterior leg. So if the sural nerve was taken for the graft, those areas would lose sensation.

Other nerves to consider: The saphenous nerve is a sensory nerve in the medial leg and foot, but the numbness described is lateral. The superficial peroneal nerve innervates the anterior and lateral leg, but the posterior leg is mentioned here. The medial plantar and lateral plantar nerves are in the foot, but not the posterior leg. The tibial nerve is more about the posterior leg but is a mixed nerve, not purely cutaneous.

So the key is that the sural nerve's sensory distribution matches the numbness described. Therefore, the correct answer is the sural nerve. The other options don't fit the areas mentioned. The clinical pearl here is that the sural nerve is a common donor for nerve grafts due to its location and function.

**Core Concept**
This question tests knowledge of peripheral nerve anatomy, specifically cutaneous nerves of the lower limb used for nerve grafting. The sural nerve is frequently harvested for grafts due to its sensory-only function and superficial location, making it a high-yield topic for surgical anatomy.

**Why the Correct Answer is Right**
The **sural nerve** is a sensory nerve formed by the union of the medial and lateral sural cutaneous nerves. It innervates the **lateral foot** and **posterior leg**, matching the described sensory deficit. Because it has no motor function, its removal causes only sensory loss, making it ideal for grafting in facial nerve repair. The numbness in the specified areas confirms its use as the graft donor.

**Why Each Wrong Option is Incorrect**
**Option A:** The *saphenous nerve* innervates the medial leg and foot, not the lateral/ posterior regions described.
**Option B:** The *superficial peroneal nerve* supplies the anterior and lateral leg but not the posterior leg.
**Option C:** The *tibial nerve* is a mixed nerve (motor and sensory) and is not purely cutaneous, making it unsuitable for grafting without significant functional loss.

**Clinical Pearl / High-Yield Fact**
The **sural nerve** is the most commonly used donor nerve for facial nerve grafts due to its sensory-only function and minimal functional

✓ Correct Answer: D. Sural