**Core Concept**
Vestibular schwannoma is a benign tumor arising from the Schwann cells of the vestibulocochlear nerve, leading to hearing loss, tinnitus, and balance disturbances. Accurate diagnosis of vestibular schwannoma is crucial for planning surgical intervention or monitoring tumor growth. The investigation of choice should provide high-resolution imaging of the internal auditory canal and cerebellopontine angle.

**Why the Correct Answer is Right**
Gadolinium-enhanced MRI is the investigation of choice for diagnosing vestibular schwannoma due to its superior soft-tissue contrast and ability to visualize the tumor's relationship with surrounding neural structures. The addition of gadolinium enhances the visibility of the tumor, allowing for accurate measurement and characterization. The high spatial resolution of MRI enables visualization of the internal auditory canal and cerebellopontine angle, where vestibular schwannoma typically arises.

**Why Each Wrong Option is Incorrect**
**Option B:** Contrast-enhanced CT is not the preferred imaging modality for vestibular schwannoma due to its lower soft-tissue contrast and inability to provide detailed visualization of the internal auditory canal and cerebellopontine angle.

**Option C:** SPECT (Single Photon Emission Computed Tomography) is not typically used for diagnosing vestibular schwannoma, as it is more suited for evaluating functional parameters such as blood flow and metabolism.

**Option D:** PET scan is not the primary imaging modality for diagnosing vestibular schwannoma, as it is more commonly used for detecting metastatic disease and monitoring treatment response.

**Clinical Pearl / High-Yield Fact**
When interpreting MRI scans for vestibular schwannoma, it's essential to assess the tumor's relationship with the facial nerve, as preservation of this nerve is crucial for maintaining facial function. A high degree of suspicion is required to detect small vestibular schwannomas, which may be difficult to visualize.

**✓ Correct Answer: A. Gadolinum enhanced MRI.**