**Core Concept**
The question is testing the student's knowledge of the anatomy of the lower limb, specifically the structures that pass deep to the medial malleolus. The medial malleolus is a bony prominence on the inner aspect of the ankle, formed by the distal end of the tibia. The structures passing deep to it are important for maintaining ankle stability and facilitating movement.

**Why the Correct Answer is Right**
The correct answer is the posterior tibial tendon, which is a strong, fibrous cord that connects the tibia to the navicular bone in the foot. It plays a crucial role in supporting the medial arch of the foot and facilitating inversion movements of the foot. The tendon passes deep to the medial malleolus, where it is protected by a groove in the tibia, known as the posterior tibial groove.

**Why Each Wrong Option is Incorrect**
**Option A:** The flexor digitorum longus tendon is a companion tendon to the posterior tibial tendon, but it passes deep to the medial malleolus at a more distal level.
**Option B:** The posterior tibial artery and vein are not structures that pass deep to the medial malleolus; rather, they are located superficial to it.
**Option C:** The saphenous nerve is a sensory nerve that runs superficial to the medial malleolus, providing sensation to the medial aspect of the leg and foot.

**Clinical Pearl / High-Yield Fact**
The posterior tibial tendon is a common site of injury, particularly in patients with flat feet or other conditions that lead to excessive stress on the tendon. Tendonitis or tears of the posterior tibial tendon can cause pain and instability in the ankle and foot.

**Correct Answer: A. Posterior tibial tendon.**