**Core Concept**
The locking of the knee joint is a condition characterized by the inability to fully extend or flex the knee due to mechanical obstruction. This is often caused by the entrapment of a loose body or a fragment within the joint space, leading to the jamming of the surrounding soft tissues.

**Why the Correct Answer is Right**
The locking of the knee joint can occur due to various reasons, but the most common causes include the presence of a loose body, such as a fragment of bone or cartilage, within the joint space. When a loose body comes into contact with the surrounding soft tissues, it can cause the joint to lock, preventing normal movement. This can be seen in conditions like osteochondral fractures, osteonecrosis, or even after a knee surgery. The loose body can be a result of a traumatic injury or a degenerative process.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is not directly related to the mechanical obstruction within the knee joint. While inflammation can cause pain and stiffness, it does not typically lead to the locking of the joint.
**Option B:** This option is incorrect as it refers to a condition that affects the ligaments surrounding the knee joint, but it does not cause mechanical obstruction or locking of the joint.
**Option C:** This option is incorrect as it refers to a condition that affects the tendons surrounding the knee joint, but it does not typically cause mechanical obstruction or locking of the joint.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the locking of the knee joint is often associated with a mechanical obstruction, such as a loose body or a fragment within the joint space. This can be a diagnostic clue to look for in patients presenting with a locked knee joint.

**Correct Answer: D. A loose body within the joint space.**