## Core Concept
The anterior compartment of the leg is one of the four compartments of the leg, along with the lateral, superficial posterior, and deep posterior compartments. It primarily contains muscles responsible for dorsiflexion of the foot and toes. The main muscles in this compartment are the tibialis anterior, extensor digitorum longus, extensor hallucis longus, and peroneus tertius.

## Why the Correct Answer is Right
The muscles listed in the options are:
- A. Tibialis anterior: This muscle is indeed located in the anterior compartment and is responsible for ankle dorsiflexion and foot inversion.
- B. Extensor digitorum longus: Also located in the anterior compartment, it aids in ankle dorsiflexion and toe extension.
- C. Extensor hallucis longus: This muscle is found in the anterior compartment as well, contributing to ankle dorsiflexion and great toe extension.
- D. Gastrocnemius: The gastrocnemius muscle is actually located in the superficial posterior compartment of the leg. It plays a key role in ankle plantarflexion.

## Why Each Wrong Option is Incorrect
- **Option A:** Tibialis anterior is a muscle of the anterior compartment, so it's not the correct answer to the question about which muscle is not in the anterior compartment.
- **Option B:** Extensor digitorum longus is also a part of the anterior compartment, making it an incorrect choice for the muscle not contained within this compartment.
- **Option C:** Extensor hallucis longus, similarly, is contained within the anterior compartment, which eliminates it as the correct answer.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the anterior compartment of the leg is prone to compartment syndrome, a condition that can arise from increased pressure within the compartment leading to muscle and nerve damage. Clinical presentation often includes pain out of proportion to the injury, pallor, pulselessness, poikilothermia (cold to touch), paresthesias, and paralysis.

## Correct Answer: D. Gastrocnemius.