The most likely diagnosis is an anterior cruciate ligament tear. Anterior cruciate ligament injury usually occurs when a person rapidly decelerates and pivots but may also develop following direct trauma that results in knee hyperextension. A complete tear should be suspected when a popping sound is reported and the patient reports pain and knee instability. The characteristic examination finding is a large effusion with increased laxity seen with both the anterior drawer and Lachman tests. In this patient, the sudden onset of knee pain, swelling, and instability; the mechanism of injury (a noncontact injury that occurred with deceleration and pivoting); and the increased laxity observed on examination with both the anterior drawer and Lachman tests all suggest a complete anterior cruciate ligament tear.

Lateral collateral ligament tears result from laterally directed (varus) forces on the knee and are associated with lateral knee pain, swelling, and instability. On examination, there is lateral joint line tenderness and increased laxity with varus-directed forces. Knee effusions are commonly seen. Although this patient has swelling and instability, which could be consistent with a lateral collateral ligament tear, he does not have lateral joint line tenderness and increased laxity with varus-directed forces, which argue against a lateral collateral ligament tear.

Medial collateral ligament tears occur as a result of a contact injury from a medially directed (valgus) force. Patients with medial collateral ligament tears typically present with medial knee pain and joint instability. On examination, there is medial joint line tenderness and increased laxity with valgus stress testing, which are not seen in this patient. A palpable knee effusion is also commonly present.

Patients with meniscal tears are typically able to bear weight immediately after the injury and are often able to continue participating in the activity they were doing before the injury, unlike this patient. Additionally, patients with meniscal tears will frequently report a locking or catching sensation. On examination, abnormal responses may be seen with both the Thessaly and medial-lateral grind tests. Knee effusions may or may not be present.