**Core Concept**
Osteochondritis dissecans is a joint condition characterized by the loosening of a piece of cartilage, along with a thin layer of the bone beneath it, from the end of a bone. This occurs due to inadequate blood supply to the affected area, leading to cartilage and bone necrosis.

**Why the Correct Answer is Right**
The most common site of osteochondritis dissecans is the knee joint, specifically the lateral aspect of the medial femoral condyle. This is because the lateral aspect of the medial femoral condyle is subjected to high stress and pressure, leading to impaired blood flow and subsequent cartilage and bone necrosis. The affected area often appears as a subchondral lesion, with the overlying cartilage intact but detached from the underlying bone.

**Why Each Wrong Option is Incorrect**
**Option A:** The elbow joint is another common site of osteochondritis dissecans, but it is less frequent than the knee joint.
**Option B:** The patellofemoral joint can also be affected, but it is not the most common site of osteochondritis dissecans.
**Option C:** The ankle joint is a rare site of osteochondritis dissecans, and it is not typically associated with this condition.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that osteochondritis dissecans often presents with pain, swelling, and locking of the affected joint, particularly in young athletes who participate in high-impact sports. A thorough history and physical examination, along with imaging studies, are crucial for diagnosing this condition.

**Correct Answer: C. The ankle joint is a rare site of osteochondritis dissecans, and it is not typically associated with this condition.