A college football player sustains a knee injury and the lower leg sags when the knee is flexed to 90 degrees. What is the most likely diagnosis?

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Multiple Choice

A college football player sustains a knee injury and the lower leg sags when the knee is flexed to 90 degrees. What is the most likely diagnosis?

Explanation:
Tibial stability in the knee is heavily dependent on the posterior cruciate ligament, which prevents the tibia from sliding backward under the femur. When the knee is bent to 90 degrees, gravity can cause the tibia to translate posteriorly if the PCL is torn, producing a posterior sag (a positive posterior drawer test). This classic sign points to a PCL injury. In contrast, an ACL tear would show anterior tibial translation, a collateral ligament sprain would cause medial or lateral instability depending on the side injured, and a meniscal tear typically presents with joint line tenderness, locking, or catching rather than a simple sag.

Tibial stability in the knee is heavily dependent on the posterior cruciate ligament, which prevents the tibia from sliding backward under the femur. When the knee is bent to 90 degrees, gravity can cause the tibia to translate posteriorly if the PCL is torn, producing a posterior sag (a positive posterior drawer test). This classic sign points to a PCL injury.

In contrast, an ACL tear would show anterior tibial translation, a collateral ligament sprain would cause medial or lateral instability depending on the side injured, and a meniscal tear typically presents with joint line tenderness, locking, or catching rather than a simple sag.

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