**Core Concept**
The knee joint is a complex synovial hinge joint that relies on the coordinated action of multiple bones, ligaments, and menisci for stability and movement. The menisci are crucial for absorbing shock, distributing weight, and facilitating smooth joint motion.

**Why the Correct Answer is Right**
The menisci are two semilunar cartilaginous structures located within the knee joint, one on either side of the medial and lateral femoral condyles. They are composed of fibrocartilage and play a vital role in reducing friction between the articulating bones, thereby preventing wear and tear on the joint. The menisci also contribute to joint stability by providing a cushioning effect during weight-bearing activities and facilitating smooth movement through the joint.

**Why Each Wrong Option is Incorrect**
**Option A:** The patella (kneecap) is a sesamoid bone located in the tendon of the quadriceps femoris muscle, which helps to protect the joint and facilitate movement, but it does not contribute to the actual structure of the knee joint.

**Option B:** The cruciate ligaments (anterior and posterior) are crucial for knee joint stability, but they are a type of ligament, not a structure that contributes to the joint's articular surface or function in the same way the menisci do.

**Option C:** The iliotibial tract is a ligament that runs down the lateral aspect of the thigh from the ilium to the tibia, providing stability to the knee joint, but it does not contribute to the joint's articular surface or function.

**Clinical Pearl / High-Yield Fact**
The menisci are often injured in sports-related activities, particularly those involving twisting or pivoting movements, and can be difficult to diagnose due to their location and the absence of clear symptoms.

**Correct Answer: C. Menisci. The menisci are two semilunar cartilaginous structures within the knee joint.**