**Core Concept**
The fallen fragment sign is a radiological feature observed in osteochondral lesions of the knee, specifically in the context of a meniscal tear. This sign is indicative of a meniscal fragment that has dislodged from its original position, often as a result of a traumatic injury.

**Why the Correct Answer is Right**
The fallen fragment sign is characterized by the presence of a small bony fragment, usually less than 1 cm in size, that has separated from the parent bone and lies within the joint space. This occurs due to the disruption of the menisco-capsular ligament, which is a critical structure that maintains the integrity of the meniscus. The presence of this sign is often associated with a bucket-handle meniscal tear, where a large fragment of the meniscus is displaced into the intercondylar notch.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the fallen fragment sign is not typically associated with osteochondritis dissecans, which is characterized by a loose body within the joint space but without the meniscal disruption.

**Option B:** This option is incorrect because the fallen fragment sign is not a feature of ligamentous injuries, such as an ACL or PCL tear, which may present with different radiological findings.

**Option C:** This option is incorrect because the fallen fragment sign is not a characteristic of articular cartilage lesions, such as a chondral defect, which may present with different radiological findings.

**Clinical Pearl / High-Yield Fact**
The fallen fragment sign is an important radiological clue that should prompt the clinician to investigate for a meniscal tear, particularly in the context of a traumatic knee injury.

**Correct Answer:** C.