A popping sensation in the knee followed by significant swelling after sprinting and stopping quickly is most consistent with which diagnosis?

• A. Medial meniscal tear
• B. ACL injury
• C. Patellar dislocation
• D. LCL sprain

Answer: (B)

A loud popping sensation at the moment of injury with rapid knee swelling after a quick sprint and sudden stop points most strongly to an ACL rupture. The ACL can tear when pivoting or decelerating forcefully, especially with a change of direction, and many athletic injuries report hearing or feeling a pop at the time of injury. The knee then fills with blood (hemarthrosis) quickly, leading to significant swelling within hours and a sense of instability. This rapid, dramatic swelling after a pivoting mechanism is the hallmark that makes an ACL tear the best fit.

Medial meniscal tears can also occur with twisting, but swelling tends to be less dramatic right away and there is often joint line tenderness or locking rather than a classic immediate hemarthrosis. Patellar dislocation presents with an obvious deformity and severe pain when trying to straighten the knee, not just a popping with rapid swelling. LCL sprains cause lateral knee pain and instability but don’t typically produce the characteristic immediate, large effusion that follows an ACL tear.