## **Core Concept**
The question tests knowledge of the anatomical structures related to the saphenous vein and the nerves in the lower limb, specifically those that could be affected during a cannulation procedure. The saphenous vein is often used for cannulation due to its superficial location. The **saphenous nerve**, a branch of the femoral nerve, accompanies the saphenous vein and provides sensory innervation to the medial aspect of the leg and foot.

## **Why the Correct Answer is Right**
The correct answer, **B. Saphenous nerve**, is right because the saphenous nerve runs alongside the saphenous vein in the leg. During a saphenous cannulation, there is a risk of accidentally ligating or damaging this nerve, leading to pain or sensory disturbances in its distribution area. The saphenous nerve provides sensory innervation to the medial border of the foot, which aligns with the patient's symptoms.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **tibial nerve** primarily provides motor innervation to the muscles of the posterior thigh and leg and sensory innervation to the sole of the foot. Damage to this nerve would not cause pain specifically along the medial border of the foot.
- **Option C:** The **peroneal nerve** (common peroneal nerve) divides into the superficial and deep peroneal nerves, which innervate the muscles of the lateral and anterior compartments of the leg, respectively. It does not provide sensation to the medial border of the foot.
- **Option D:** The **dorsal cutaneous nerve** of the foot, usually branches of the peroneal nerves, provides sensation to the dorsum of the foot, not the medial border.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the saphenous nerve is at risk during procedures involving the saphenous vein due to its close proximity. Clinically, **saphenous nerve injury** can lead to **Morton’s neuroma-like symptoms** or simply pain and sensory changes in its distribution area. Knowledge of this nerve's course can help prevent such complications during venous cannulation.

## **Correct Answer:** B. Saphenous nerve