**Core Concept**
The deltoid ligament is a strong, fibrous ligament that provides medial (inner) ankle stability. It consists of four distinct bands and plays a crucial role in resisting inversion (inward rolling) of the ankle. Injuries to this ligament can occur due to excessive twisting or rolling of the ankle, often resulting in significant pain and swelling.

**Why the Correct Answer is Right**
The deltoid ligament is located on the medial aspect of the ankle joint and is composed of four distinct bands: the tibionavicular, tibiocalcaneal, anterior tibiotalar, and posterior tibiotalar bands. Given the patient's history of a twisting injury and the presence of significant swelling around the medial malleolus, it is likely that the deltoid ligament has been injured. This type of injury is often referred to as an "anterior drawer" or "talar tilt" due to the abnormal movement of the talus relative to the tibia.

**Why Each Wrong Option is Incorrect**
**Option B:** The anterior talofibular ligament (ATFL) is a ligament that provides lateral (outer) ankle stability and is often injured in ankle sprains. However, the ATFL is located on the lateral aspect of the ankle, not the medial aspect, making it an unlikely candidate for injury in this case.

**Option C:** The spring ligament is a ligament that supports the head of the talus and helps maintain the medial longitudinal arch of the foot. While it can be injured, this ligament is not typically associated with medial ankle injuries.

**Option D:** The Achilles tendon is a tendon that connects the calf muscles to the calcaneus (heel bone) and is not directly related to ankle injuries in the medial aspect.

**Clinical Pearl / High-Yield Fact**
In ankle injuries, it's essential to distinguish between medial (deltoid ligament) and lateral (ATFL) injuries, as the treatment and prognosis can differ significantly. A thorough physical examination and imaging studies, such as X-rays or MRI, can help determine the extent and location of the injury.

**✓ Correct Answer: A. Deltoid Ligament**