**Core Concept**
Cholesteatoma is a destructive, expanding lesion of the middle ear that can cause bone erosion through enzymatic action on surrounding osseous structures. The extent of destruction depends on the location and size of the cholesteatoma.

**Why the Correct Answer is Right**
The basal plate of the stapes is not at immediate risk of erosion by cholesteatoma because it is not in direct contact with the cholesteatoma. Cholesteatomas typically arise in the attic or posterior-superior quadrant of the middle ear and expand laterally, eroding the long process of the incus and the horizontal semicircular canal. The facial nerve, which is housed in the Fallopian canal, is also at risk due to its proximity to the cholesteatoma. However, the basal plate of the stapes is located more medially and is less likely to be affected directly by the expanding lesion.

**Why Each Wrong Option is Incorrect**
**Option A:** The long process of the incus is a common site for cholesteatoma erosion due to its proximity to the attic region of the middle ear.

**Option B:** The Fallopian canal containing the facial nerve is at risk of erosion by cholesteatoma due to its close proximity to the expanding lesion.

**Option C:** The horizontal semicircular canal is also at risk of erosion by cholesteatoma due to its location in the posterior-superior quadrant of the middle ear.

**Clinical Pearl / High-Yield Fact**
Remember that cholesteatomas can cause significant bone erosion, but the extent of destruction depends on the location and size of the lesion. Early detection and treatment are crucial to preventing complications such as hearing loss, facial paralysis, and intracranial extension.

**✓ Correct Answer: D. Basal plate of stapes**