**Core Concept**
Acoustic neuromas, also known as vestibular schwannomas, are benign tumors arising from the Schwann cells of the vestibular nerve. They can compress adjacent cranial nerves, leading to various symptoms. The early involvement of cranial nerves is crucial in determining the clinical presentation and prognosis of patients with acoustic neuromas.

**Why the Correct Answer is Right**
The trigeminal nerve (CN V), particularly its fifth branch (CN V2 and CN V3), is often the first cranial nerve to be compressed by an acoustic neuroma. This occurs because the trigeminal nerve is closely related to the cerebellopontine angle, where acoustic neuromas typically arise. Compression of CN V can lead to symptoms such as facial numbness, pain, and weakness, which are often early warning signs of an acoustic neuroma. The trigeminal nerve's early involvement is due to its close proximity to the tumor and its relatively small size, making it more susceptible to compression.

**Why Each Wrong Option is Incorrect**
**Option B:** CN VII (facial nerve) is often involved later in the progression of acoustic neuromas, as it is more lateral than CN V and CN VIII (cochlear nerve).
**Option C:** CN X (vagus nerve) is not typically affected early in acoustic neuromas, as it is located more medially and is not as closely related to the cerebellopontine angle.
**Option D:** CN IX (glossopharyngeal nerve) is also not commonly involved early in acoustic neuromas, as it is located more medially and is not as closely associated with the tumor.

**Clinical Pearl / High-Yield Fact**
In patients with acoustic neuromas, the presence of trigeminal nerve symptoms (e.g., facial numbness or pain) should prompt further investigation, as these symptoms can indicate early involvement of the tumor and necessitate timely intervention.

✓ Correct Answer: A. CN 5