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O'Donahue's triad, also known as the "unhappy triad," typically involves three structures that are commonly injured in knee injuries, particularly those that occur during sports. The triad consists of injuries to the medial collateral ligament (MCL), the anterior cruciate ligament (ACL), and the medial meniscus.

In this context, the medial collateral ligament is singled out because it is frequently damaged due to the nature of the mechanisms of injury associated with the triad. When a lateral force is applied to the knee while the foot is planted, the MCL is put under stress, often leading to sprains or tears.

The other structures listed, while important to knee stability and function, are not part of the classic triad. The patellar tendon is involved in the extensor mechanism of the knee, the Achilles tendon is related to the ankle and foot motion, and the hamstring muscles are primarily involved in hip extension and knee flexion. Therefore, the medial collateral ligament is the most commonly injured structure in O'Donahue's triad, making it the correct answer.