**Core Concept**
The ankle joint is stabilized by a complex set of ligaments, which are crucial for maintaining joint integrity and preventing excessive movement. Ankle sprains often occur due to forced inversion of a plantarflexed foot, leading to ligamentous injury.

**Why the Correct Answer is Right**
Forced inversion of a plantarflexed foot typically results in injury to the ligaments on the lateral (outer) aspect of the ankle joint. The anterior talofibular ligament (ATFL) is the most commonly injured ligament in this scenario. This occurs because the ATFL is located on the front of the lateral ankle, making it susceptible to stretching or tearing when the foot is inverted. The ATFL plays a vital role in preventing excessive inversion of the ankle joint.

**Why Each Wrong Option is Incorrect**
**Option B:** The posterior talofibular ligament (PTFL) is located on the back of the lateral ankle and is not typically injured in the setting of forced inversion of a plantarflexed foot.

**Option C:** The calcaneofibular ligament (CFL) is also located on the lateral ankle, but it is not the primary ligament injured in this scenario. While the CFL can be injured in severe cases, the ATFL is more commonly affected.

**Option D:** The posterior fibers of the deltoid ligament are located on the medial (inner) aspect of the ankle joint and are not typically injured in the setting of forced inversion of a plantarflexed foot.

**Clinical Pearl / High-Yield Fact**
It's essential to note that ankle sprains can be categorized based on the severity of the injury, with grade 1 being a mild sprain, grade 2 being a moderate sprain, and grade 3 being a severe sprain. The ATFL is often the first ligament to be injured in ankle sprains, and its integrity is crucial for preventing further injury to the surrounding ligaments.

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