**Core Concept**
The knee joint is a complex hinge joint with both bony and ligamentous stabilizers. When the knee is fully extended, the tension in the ligaments and the alignment of the bones change, affecting the joint's stability and proprioception.

**Why the Correct Answer is Right**
When a healthy young athlete extends their knee from 90 degrees of flexion to full extension, the quadriceps muscle contracts to straighten the knee. This action increases the tension in the anterior cruciate ligament (ACL), which is responsible for stabilizing the knee joint. The ACL prevents excessive anterior translation of the tibia relative to the femur. As the knee extends, the tension in the ACL increases due to the increased angle between the tibia and the femur, ensuring joint stability.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as the question does not provide enough information to suggest a meniscal injury. Meniscal injuries typically occur due to a twisting or pivoting motion of the knee, not a straightforward extension.
* **Option B:** This option is incorrect as the patellofemoral joint is not directly affected by the extension of the knee joint. The patellofemoral joint is responsible for the movement of the patella within the femoral groove.
* **Option C:** This option is incorrect as the posterior cruciate ligament (PCL) is not directly affected by the extension of the knee joint. The PCL prevents excessive posterior translation of the tibia relative to the femur, which is not relevant in this scenario.

**Clinical Pearl / High-Yield Fact**
When assessing knee injuries, it's essential to remember the "ABCDE" of knee examination: alignment, bulk, congruity, movement, and end feel. This systematic approach helps to identify potential injuries and guide further investigation.

**Correct Answer:** A.