**Core Concept**
MacEwen's triangle is an anatomical structure in the skull that refers to the area bounded by the superior temporal line, the posterior root of the zygomatic process of the temporal bone, and the ligament connecting the two. This triangle is relevant in neurosurgery and otolaryngology for surgical approaches to the temporal bone and surrounding structures.

**Why the Correct Answer is Right**
The boundaries of MacEwen's triangle are formed by the superior temporal line, the posterior root of the zygomatic process of the temporal bone, and the ligament connecting the two. The superior temporal line is a bony ridge along the temporal bone, while the posterior root of the zygomatic process of the temporal bone is a bony projection. The ligament connecting the two forms the base of the triangle. This anatomical landmark is crucial for surgeons to understand when performing procedures in the temporal region.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the superior temporal line is indeed one of the boundaries of MacEwen's triangle.
**Option B:** This option is incorrect because the posterior root of the zygomatic process of the temporal bone is also one of the boundaries of MacEwen's triangle.
**Option D:** This option is incorrect because the ligament connecting the two boundaries of MacEwen's triangle is the actual third boundary.

**Clinical Pearl / High-Yield Fact**
The boundaries of MacEwen's triangle are essential for surgeons to recognize, as it provides a safe and accessible approach to the temporal bone and surrounding structures. A thorough understanding of this anatomical landmark is crucial for successful surgical outcomes.

**Correct Answer: A.**