**Core Concept**
The ankle joint is stabilized by multiple ligaments, including the anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), posterior talofibular ligament (PTFL), and deltoid ligament. The ATFL is a critical structure that resists inversion stress, making it prone to injury in ankle sprains.

**Why the Correct Answer is Right**
The ATFL is the most commonly injured ligament in ankle sprains due to its anatomical location and function. It is a strong, yet thin ligament that is subjected to significant stress during inversion injuries. The ATFL is attached to the anterior margin of the fibula and the posterior margin of the talus, making it vulnerable to tears and strains.

**Why Each Wrong Option is Incorrect**
* **Option A:** The calcaneofibular ligament (CFL) is also injured in ankle sprains, but it is less common than the ATFL. The CFL is located more posteriorly than the ATFL and is more resistant to inversion stress.
* **Option B:** The posterior talofibular ligament (PTFL) is a strong ligament that provides stability to the ankle joint, but it is less commonly injured than the ATFL. The PTFL is located more posteriorly than the ATFL and is more resistant to inversion stress.
* **Option C:** The deltoid ligament is a complex ligament that provides stability to the medial aspect of the ankle joint. While it can be injured in severe ankle sprains, it is less common than ATFL injuries.
* **Option D:** The Achilles tendon is a tendon that connects the calf muscles to the calcaneus, not a ligament that is commonly injured in ankle sprains.

**Clinical Pearl / High-Yield Fact**
The ATFL is often injured in combination with the CFL, making it essential to assess both ligaments in patients with ankle sprains. A thorough physical examination, including the anterior drawer test and the squeeze test, can help diagnose ATFL injuries.

**Correct Answer: A. Anterior talofibular ligament**