Which of the following tests is used to assess knee instability?

The Lachman's test is specifically designed to assess knee instability, particularly for evaluating anterior cruciate ligament (ACL) integrity. During this test, the examiner stabilizes the femur with one hand while applying an anterior force to the tibia with the other hand. This movement assesses the degree of forward movement of the tibia relative to the femur, which can indicate ACL damage or instability in the knee joint. If there is excessive anterior translation of the tibia with respect to the femur, it suggests ACL injury, indicating knee instability.

While McMurray's test and the Valgus stress test also evaluate knee stability and integrity, they have different specific focuses. McMurray's test is primarily used to detect meniscal tears, and the Valgus stress test assesses the integrity of the medial collateral ligament (MCL). Though these tests relate to knee stability, the Lachman's test is the most definitive for determining ACL-related instability. Hence, it is essential to identify the correct test that targets knee instability most directly, which is why the Lachman's test is the appropriate answer in this context.