**Core Concept**
The anterior compartment of the leg contains muscles responsible for dorsiflexion of the foot and toe flexion. This compartment is bounded by the interosseous membrane and the anterior border of the tibia and fibula.

**Why the Correct Answer is Right**
The correct answer, Peroneus tertius, is indeed a muscle located in the anterior compartment of the leg. It is a long, thin muscle that arises from the anterior border of the fibula and inserts into the base of the fifth metatarsal bone. The primary function of the Peroneus tertius is to assist in dorsiflexion of the foot, particularly when the foot is in a plantarflexed position. This is achieved through its innervation by the deep peroneal nerve (L5 and S1 nerve roots). The Peroneus tertius works synergistically with the tibialis anterior, extensor digitorum longus, and extensor hallucis longus muscles to facilitate dorsiflexion of the foot.

**Why Each Wrong Option is Incorrect**
**Option A:** Peroneus brevis is actually located in the lateral compartment of the leg, which is responsible for foot eversion. It is innervated by the superficial peroneal nerve.
**Option B:** Peroneus longus is also located in the lateral compartment of the leg and is primarily responsible for foot eversion and support of the arch. It is innervated by the superficial peroneal nerve.
**Option D:** Flexor digitorum longus is a muscle located in the posterior compartment of the leg, which is responsible for flexion of the toes. It is innervated by the tibial nerve (S2-S3 nerve roots).

**Clinical Pearl / High-Yield Fact**
The anterior compartment of the leg is often referred to as the "dorsiflexor" compartment due to the primary function of its muscles in dorsiflexing the foot. It is essential for maintaining balance and preventing falls.

✓ Correct Answer: A. Peroneus tertius