**Core Concept**
The medial meniscus is a C-shaped fibrocartilaginous structure in the knee joint, located between the medial femoral condyle and the medial tibial plateau. It plays a crucial role in absorbing shock, facilitating smooth movement, and distributing loads during weight-bearing activities.

**Why the Correct Answer is Right**
Injury to the medial meniscus is less likely when the knee joint is in a position where the medial meniscus is naturally protected or displaced. This occurs when the knee is in full extension, as the meniscus is tucked under the femoral condyle and is less susceptible to injury. In this position, the medial meniscus is not in close proximity to the tibial plateau, reducing the risk of meniscal tears.

**Why Each Wrong Option is Incorrect**
* **Option A:** Injury to the medial meniscus is possible in flexion, as the meniscus can become trapped between the femur and tibia, increasing the risk of tears.
* **Option B:** Hyperextension of the knee joint can lead to meniscal injuries, including tears of the medial meniscus, as the meniscus is stretched and subjected to abnormal stresses.
* **Option C:** Valgus stress, or lateral deviation of the knee, can also lead to medial meniscal injuries, as the meniscus is compressed and subjected to abnormal forces.

**Clinical Pearl / High-Yield Fact**
The "bucket-handle" tear of the medial meniscus typically occurs when the meniscus is torn and displaced into the intercondylar notch, often in association with a flexion injury.

**Correct Answer:** D. Full extension