**Core Concept**
The sustentaculum tali is a bony projection on the medial aspect of the calcaneus that serves as a support for the talus bone. It is an important anatomical landmark in the foot, particularly in relation to the Achilles tendon and the flexor hallucis longus tendon.

**Why the Correct Answer is Right**
The flexor hallucis longus tendon passes below the sustentaculum tali as it courses from the posterior aspect of the tibia to the distal phalanx of the great toe. This allows the tendon to pass under the calcaneus and reach its insertion site without being impinged by the sustentaculum tali. The flexor hallucis longus muscle is responsible for plantarflexion of the great toe, which is an essential component of balance and gait.

**Why Each Wrong Option is Incorrect**
**Option A:** The Achilles tendon passes over the posterior aspect of the calcaneus, not below the sustentaculum tali. It is a thick, fibrous tendon that connects the gastrocnemius and soleus muscles to the calcaneus, facilitating plantarflexion of the foot.
**Option B:** The tendons of the flexor digitorum longus and flexor hallucis brevis do not pass below the sustentaculum tali. These tendons are involved in plantarflexion and flexion of the toes, respectively, but they have different courses and insertions.
**Option C:** The posterior tibial tendon passes above the sustentaculum tali, not below it. This tendon supports the medial arch of the foot and helps to prevent flatfoot deformities.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with foot pain or weakness, it is essential to consider the anatomy of the tendons and their relationships to the sustentaculum tali. Knowledge of these anatomical relationships can help guide diagnosis and treatment of conditions such as Achilles tendonitis, flexor hallucis longus tendonitis, and posterior tibial tendonitis.

**Correct Answer: C. The posterior tibial tendon passes above the sustentaculum tali, not below it. This tendon supports the medial arch of the foot and helps to prevent flatfoot deformities.**