The most likely injured structure in a skier's knee following a valgar rotation injury is the:

• A. Medial meniscus
• B. Patellar tendon
• C. Anterior cruciate ligament (ACL)
• D. Posterior cruciate ligament (PCL)

Answer: (C)

In a skier's knee injury that involves a valgus (meaning an outward angulation of the distal segment of a bone or joint) rotation, the anterior cruciate ligament (ACL) is the most commonly injured structure. This type of injury typically occurs when the knee is forced into a valgus position while simultaneously rotating the body, which places significant stress on the ACL. The ACL is crucial for maintaining knee stability and controlling movement, particularly in activities that involve sudden changes in direction or deceleration, which is common in skiing.

In this scenario, the mechanism of injury leads to excessive strain on the ACL, making it more susceptible to tears or other forms of damage. While the medial meniscus can also be injured during a valgus knee injury, particularly in conjunction with ACL injuries, the primary structure that is compromised during this specific mechanism is the ACL. The patellar tendon and posterior cruciate ligament (PCL) are less commonly injured due to the specific nature of the valgus rotation injury described.

The emphasis here is on the specific mechanics of the injury as they relate to the function and anatomical positioning of the ACL within the knee joint, highlighting its vulnerability during such traumata.