During a football game a 21-year-old wide receiver was illegally blocked by a linebacker, who threw himself against the posterolateral aspect of the runner’s left knee. As he lay on the ground, the wide receiver grasped his knee in obvious pain. Which of the following structures is frequently subject to injury from this type of force against the knee?

**Core Concept**
The posterolateral aspect of the knee is susceptible to injury from forces directed against the lateral aspect of the knee, particularly the collateral ligaments and the posterolateral corner structures. This is due to the biomechanics of the knee joint, where forces directed laterally can cause stress on the lateral collateral ligament (LCL) and the posterolateral corner (PLC) structures.

**Why the Correct Answer is Right**
When a force is applied to the posterolateral aspect of the knee, it can cause strain or avulsion injuries to the lateral collateral ligament (LCL) and the posterolateral corner (PLC) structures, including the popliteus tendon, the popliteofibular ligament, and the fibular collateral ligament. The PLC is particularly vulnerable due to its anatomical location and the fact that it provides stability to the knee joint against external rotation and varus forces.

**Why Each Wrong Option is Incorrect**
**Option A:** The medial collateral ligament (MCL) is more commonly injured by valgus forces directed against the medial aspect of the knee, not the posterolateral aspect.
**Option B:** The anterior cruciate ligament (ACL) is more commonly injured by non-contact mechanisms, such as landing from a jump or sudden changes in direction, rather than by direct contact.
**Option C:** The posterior cruciate ligament (PCL) is more commonly injured by direct blows to the anterior aspect of the tibia, such as in a dashboard injury, rather than by forces directed against the posterolateral aspect of the knee.

**Clinical Pearl / High-Yield Fact**
The PLC is a common source of knee instability and pain, particularly in contact sports, and should be considered in the differential diagnosis of knee injuries, especially in the setting of posterolateral trauma.

**Correct Answer:** .

✓ Correct Answer: B. Anterior cruciate ligament