**Core Concept**
The mastoid antrum is a small air-filled cavity located in the mastoid part of the temporal bone, directly posterior to the tympanic membrane. Its lateral wall is not covered by bone but rather by a specific anatomical region that serves as a landmark for surgical approaches and imaging.

**Why the Correct Answer is Right**
The suprameatal triangle is a triangular area located superior to the external auditory meatus, formed by the superior and posterior margins of the external auditory meatus and the mastoid process. It overlies the lateral wall of the mastoid antrum and is clinically significant as it contains the external auditory meatus and the posterior aspect of the tympanic membrane. The mastoid antrum lies just deep to this region, and the suprameatal triangle acts as a surface landmark for the underlying bony structures.

**Why Each Wrong Option is Incorrect**
Option A: Tegmen tympani is a thin layer of bone that separates the middle ear from the cranial cavity, forming the posterior wall of the tympanic cavity, not the lateral wall of the mastoid antrum.
Option B: The mastoid process is a bony projection of the temporal bone posterior to the ear, forming the posterior aspect of the mastoid antrum, not overlying its lateral wall.
Option C: The tympanic plate is part of the tympanic membrane and lies within the middle ear cavity, not overlying the mastoid antrum.

**Clinical Pearl / High-Yield Fact**
The suprameatal triangle is a key surgical landmark in procedures like mastoidectomy and tympanoplasty, and its borders help identify the proximity of the mastoid antrum to the external auditory meatus. Remember: "Suprameatal triangle → overlying lateral wall of mastoid antrum."

✓ Correct Answer: D. Suprameatal triangle