A 22-year-old male soccer player is forced to leave the game following a head-to-head collision with another player. He is admitted to the hospital, and radiographic examination reveals slight dislocation of the atlantoaxial joint. As a result, he experiences decreased range of motion at that joint. What movement of the head would most likely be severely affected?

**Core Concept**
The atlantoaxial joint is a synovial joint located in the cervical spine, specifically between the atlas (C1) and axis (C2) vertebrae. It allows for a wide range of motion in the head, facilitating nodding, shaking, and other rotational movements. The joint's stability is maintained by the transverse ligament of the atlas and the alar ligaments.

**Why the Correct Answer is Right**
The atlantoaxial joint is responsible for rotational movements of the head, particularly the ability to turn the head from side to side. The joint's slight dislocation would most likely affect the rotational movement of the head, particularly lateral flexion and rotation. This is because the atlantoaxial joint is primarily involved in facilitating these movements. The transverse ligament of the atlas, which is damaged in this patient, plays a crucial role in maintaining the joint's stability and facilitating rotational movements.

**Why Each Wrong Option is Incorrect**
**Option A:** Extension and flexion movements are primarily facilitated by the atlantooccipital joint, not the atlantoaxial joint. Although the atlantoaxial joint does contribute to these movements, the atlantooccipital joint is more responsible for extension and flexion.

**Option B:** Torsional movements are not a primary function of the atlantoaxial joint. While the joint does allow for some degree of torsion, it is not the primary movement affected by a dislocation of the atlantoaxial joint.

**Option C:** Rotation is indeed a movement affected by a dislocation of the atlantoaxial joint, but the more specific movement is lateral flexion and rotation. The atlantoaxial joint is primarily involved in facilitating these movements.

**Option D:** Anteflexion is not a movement facilitated by the atlantoaxial joint. This movement is more closely associated with the thoracic spine.

**Clinical Pearl / High-Yield Fact**
The transverse ligament of the atlas plays a crucial role in maintaining the stability of the atlantoaxial joint and facilitating rotational movements. Injury to this ligament can lead to atlantoaxial instability and subsequent neurological deficits.

**Correct Answer:** C. Rotation.

✓ Correct Answer: A. Rotation