A 22-year-old male basketball player presents with a swollen, painful left knee. He was on a fast break and as he began to jump for an alley-oop, he felt his knee give out and he collapsed to the floor. What is the most likely diagnosis
**Core Concept**
The patient's presentation of a swollen, painful knee following a sudden twisting injury is suggestive of a ligamentous injury, specifically a tear of the anterior cruciate ligament (ACL). The ACL is a critical stabilizer of the knee joint, and its disruption can lead to knee instability and pain.
**Why the Correct Answer is Right**
The mechanism of injury, a sudden twisting motion, is consistent with an ACL tear. When the knee is subjected to a valgus force (a force that pushes the knee inward), the ACL is stretched and may tear. This injury is common in sports that involve quick changes of direction, such as basketball. The ACL is a ligament that connects the femur (thigh bone) to the tibia (shin bone) and provides stability to the knee joint.
**Why Each Wrong Option is Incorrect**
* **Option A:** Meniscal tears are also common in knee injuries, but they typically occur with a bending or twisting motion, rather than a sudden twisting motion as described.
* **Option B:** Medial collateral ligament (MCL) injuries occur with a valgus force, but they typically present with pain on the medial aspect of the knee, rather than a swollen, painful knee.
* **Option D:** Osteochondral fractures occur with a direct blow to the knee, rather than a twisting motion.
**Clinical Pearl / High-Yield Fact**
The "Lachman test" is a sensitive test for ACL tears, where the examiner holds the patient's knee at 20-30 degrees of flexion and applies an upward force to the tibia while stabilizing the femur. A positive test is suggestive of an ACL tear.
**Correct Answer: C. ACL tear**