**Question:** A 25-year-old man experiences pain in the right knee while skiing, causing his knee to twist and him to fall to the ground. His knee is swollen. He cannot bear full weight or fully extend or bend his leg. There is tenderness over the medial joint line (Figure below). Emergency-room x-ray findings were normal, and the range of motion (ROM), although restricted, is stable to varus and valgus stress. Straight-leg raise is unrestricted. Which is the most likely type of injury?

A. Anterior Cruciate Ligament (ACL) tear
B. Medial Collateral Ligament (MCL) tear
C. Meniscal tear
D. Patellar dislocation

**Correct Answer:**

**Core Concept:** The scenario describes a patient with a knee injury presenting with swelling, pain, and restricted range of motion, but stable to varus and valgus stress and an unrestricted straight-leg raise. These symptoms and findings suggest an injury involving the ligaments on the medial side of the knee.

**Why the Correct Answer is A:**

The correct answer is A, Anterior Cruciate Ligament (ACL) tear, due to the specific clinical presentation and findings:

1. The patient reports pain over the medial joint line, which is consistent with the ACL's location.
2. The restriction in full extension and flexion, but stable to varus and valgus stress, points towards a medial structure injury.
3. An unrestricted straight-leg raise suggests that the injury is not involving the knee joint (e.g., meniscal tear) or patella (e.g., dislocation).

**Why Other Options are Incorrect:**

Let's briefly discuss the other options:

**Option B (Medial Collateral Ligament [MCL] tear):**
1. An MCL injury usually presents with more diffuse pain and less severe locking or giving-way sensation compared to the described case, which shows more specific medial joint line pain.
2. MCL tears are usually associated with lateral joint line pain and instability, not medial joint line pain and restricted flexion and extension.

**Option C (Meniscal tear):**
1. A meniscal tear typically presents with a more diffuse knee pain, often associated with locking or giving-way sensation, not specific medial joint line pain as in the described case.
2. Meniscal tears usually cause more lateral joint line pain and less severe locking or giving-way sensation compared to the described case.

**Option D (Patellar dislocation):**
1. A patellar dislocation presents with knee instability, not the specific medial joint line pain and restricted flexion and extension observed in the described case.
2. Dislocated patella causes more anterior knee pain and instability, not medial joint line pain and restricted flexion and extension as in the described case.

In summary, the correct answer is A, Anterior Cruciate Ligament (ACL) tear, due to the specific medial joint line pain, restricted flexion and extension, and an unrestricted straight