**Core Concept**
Cholesteatoma is a type of skin cyst located in the middle ear, which can lead to erosion of surrounding structures due to its aggressive nature and the continuous growth of keratin-producing cells. This can result in severe complications, including facial nerve paralysis and intracranial complications.

**Why the Correct Answer is Right**
The ossicles, particularly the malleus and incus, are at immediate risk of erosion by cholesteatoma due to their proximity to the epitympanic recess where cholesteatomas typically form. However, the petrous apex is less commonly affected, as cholesteatomas tend to grow laterally towards the attic and the middle ear cavity. Therefore, the petrous apex is not at immediate risk of erosion.

**Why Each Wrong Option is Incorrect**
**Option A:** The ossicles are indeed at risk, particularly the malleus and incus, due to their proximity to the epitympanic recess.
**Option B:** The facial nerve is at risk of paralysis due to its proximity to the cholesteatoma and its potential erosion.
**Option C:** The lateral semicircular canal is at risk of erosion and subsequent perilymphatic fistula formation.

**Clinical Pearl / High-Yield Fact**
Cholesteatomas can be classified into primary and secondary types. Primary cholesteatomas occur in the absence of any known middle ear disease, while secondary cholesteatomas occur in association with a pre-existing middle ear disease, such as otosclerosis or chronic otitis media.

**Correct Answer:** D.