Okay, let's tackle this question about the "Double PCL" sign on MRI of the knee. First, I need to recall what the PCL is. The posterior cruciate ligament is one of the major ligaments in the knee, connecting the femur to the tibia. The "Double PCL" sign sounds like a specific MRI finding.

I remember that certain pathologies can cause changes in the appearance of ligaments. For example, in meniscal cysts or Baker's cysts, there might be fluid collections. But the PCL itself could be affected in conditions like avulsion injuries or degenerative changes. Wait, avulsion of the PCL would mean the ligament is torn at its attachment point. If the ligament is torn, on MRI, you might see a gap or abnormal signal. The "Double PCL" sign might refer to the appearance of two structures where there should be one, perhaps due to a hematoma or a cyst adjacent to the PCL.

Alternatively, in cases of a meniscal cyst communicating with the joint, especially the medial meniscus, the cyst could be near the PCL. On MRI, this might create a "double" image because of the cyst adjacent to the normal PCL. Another possibility is a popliteal cyst (Baker's cyst), which is a fluid-filled cyst in the popliteal fossa. If it's close to the PCL, it might mimic a second ligament on imaging.

Wait, but the question is about the "Double PCL" sign specifically. Let me think. A popliteus tendon tear? No, that's a different structure. The PCL has two bundles: the anterior and posterior, but the sign is called "double," which suggests an abnormality.

I think the correct answer is a meniscal cyst, particularly the medial meniscus, which can form a cyst that appears adjacent to the PCL on MRI, creating the appearance of a double PCL. This is a classic sign. So the correct answer would be related to meniscal cysts. Let me verify.

Yes, the "Double PCL" sign is associated with a medial meniscal cyst that is adjacent to the posterior cruciate ligament. The cyst appears as a separate structure next to the PCL, giving the MRI the appearance of two PCLs. So the correct answer should be an option that mentions a medial meniscal cyst.

Now, the incorrect options might include Baker's cyst, PCL avulsion, or other ligament injuries. But the key is the "double" appearance, which is specific to the meniscal cyst next to the PCL. The clinical pearl is to remember that the "Double PCL" sign is a high-yield image for medial meniscal cysts, which are often associated with degenerative changes or tears in the meniscus.

**Core Concept**
The "Double PCL" sign on MRI refers to the imaging appearance of a medial meniscal cyst adjacent to the posterior cruciate ligament (PCL), creating a pseudoduplication of the ligament. This occurs due to fluid-filled cysts communicating with the knee joint, typically associated with degenerative meniscal tears.

**Why the Correct Answer is Right**
A medial meniscal cyst, often arising from a degenerative tear in the medial meniscus, can form a fluid-filled cavity

✓ Correct Answer: B. Medial meniscus tear