## **Core Concept**
The question tests the understanding of knee injuries, specifically meniscal tears, ligament sprains, and other knee pathologies. The clinical presentation and diagnostic findings point towards a meniscal injury. The **menisci** are crucial for knee joint stability and function, and their injuries are common in sports and trauma.

## **Why the Correct Answer is Right**
The patient's symptoms, such as pain following a fall, swelling, inability to bear full weight or fully extend the leg, and tenderness over the **medial joint line**, are classic for a **meniscal tear**. The normal X-ray findings help rule out fractures. The stability to varus and valgus stress suggests that the major ligaments (like ACL, PCL, MCL, LCL) are likely intact. The unrestricted straight-leg raise indicates no significant hamstring or quadriceps inhibition, often seen in more severe injuries or significant pain. A meniscal tear, particularly a **medial meniscal tear**, fits well with these findings.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, choices might include other knee injuries like ligament sprains or fractures, which are less likely given the clinical scenario.
- **Option B:** Similarly, without specifics, if this option suggested a different type of injury such as a cruciate ligament injury, it would be less likely due to the stability on varus and valgus stress and the specific location of tenderness.
- **Option C:** If this option suggested a fracture, it would be incorrect due to the normal X-ray findings.
- **Option D:** If this suggested another type of soft tissue injury not consistent with the medial joint line tenderness and inability to fully extend, it would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical test for meniscal injuries is the **McMurray test**, which involves flexion and rotation of the knee to assess for pain or clicking, indicative of a meniscal tear. However, the question focuses on clinical presentation and stability, pointing towards a meniscal injury without needing specific test results.

## **Correct Answer:** . Meniscal tear.