Describe the most common mechanism of an ACL injury and two typical clinical signs.

• A. Direct contact injury to knee.
• B. Noncontact pivoting with knee valgus during deceleration; signs: a loud pop with swelling.
• C. Noncontact pivoting with valgus during deceleration; signs: a loud pop or sudden swelling.
• D. Twisting injury with foot planted causing calf strain.

Answer: (B)

Most ACL injuries happen without direct contact when the foot is planted and the knee pivots while decelerating, often with the knee collapsing into valgus. Two classic clinical signs accompany this mechanism: an audible pop at the time of injury and rapid swelling of the knee within hours due to intra-articular bleeding (hemarthrosis). This combination fits best because it describes the common noncontact pivoting with valgus during deceleration and the two hallmark signs that many patients report. Direct contact injuries are less typical for ACL tears, a calf strain describes a different tissue injury, and using “or” for the signs in another option doesn’t reflect the common pairing of a loud pop and swift swelling.