American Board of Internal Medicine (ABIM) Certification Practice Exam

What finding suggests a complete tear of the anterior cruciate ligament?

1. Joint line tendernes

2. A popping sound with knee instability

3. Pes anserine tenderness

4. Swelling on the medial side

The correct answer is: A popping sound with knee instability

A popping sound accompanied by knee instability is indicative of a complete tear of the anterior cruciate ligament (ACL). This specific finding arises during the injury event when the ligament is torn, often described as a "pop." The sudden onset of knee instability following such an event is characteristic of a complete rupture, as the ACL is essential for stabilizing the knee during movement, particularly in activities that involve sudden changes in direction or pivoting. In particular, the presence of instability is crucial; patients often report a sensation of the knee "giving way" or feeling unstable, confirming the loss of normal knee mechanics due to the compromised ACL. This symptom, combined with the auditory feedback of a popping sound, helps differentiate a complete ACL tear from other knee injuries, which may present with different signs or symptoms. Other options may indicate various knee injuries or conditions but do not specifically denote a complete tear of the ACL. For instance, joint line tenderness is more associated with meniscal injuries, pes anserine tenderness relates to issues in the tendons of the pes anserinus, and swelling on the medial side can occur due to various injuries or conditions affecting the knee but is not specific to an ACL tear.