**Core Concept**
The temporomandibular joint (TMJ) is a synovial joint with specific structural elements that enable mandibular movement. Key components include the articular disc, joint cavities, and ligaments that provide stability and facilitate smooth motion.

**Why the Correct Answer is Right**
The sigmoid notch is a bony feature of the temporal bone, located on the lateral aspect of the skull, and serves as the articulation site for the mandibular condyle. However, it is not a *structural element* of the TMJ itself but rather a part of the skull. The articular disc (B), capsular ligament (C), and joint cavities (D) are all actual anatomical components of the TMJ that directly contribute to joint function and stability.

**Why Each Wrong Option is Incorrect**
Option B: The articular disc is a fibrocartilaginous structure that separates the condyle from the skull and acts as a cushion for joint movement. It is a core structural element of the TMJ.
Option C: The capsular ligament surrounds the joint and provides stability during opening and closing of the mouth. It is a key structural component.
Option D: The joint cavities (also called the articular space) are enclosed spaces within the TMJ that house the joint surfaces and allow for movement. They are essential structural features.

**Clinical Pearl / High-Yield Fact**
Remember: The **sigmoid notch** is part of the temporal bone, not a component of the TMJ structure itself. Always differentiate between bony landmarks and true joint elements—this is a common exam trap.

✓ Correct Answer: A. Sigmoid notch