**Core Concept**
Vestibular schwannoma, also known as acoustic neuroma, is a benign tumor originating from the Schwann cells of the vestibular nerve, which is responsible for balance and equilibrium. The diagnosis of vestibular schwannoma requires a combination of clinical evaluation and imaging studies to differentiate it from other causes of unilateral hearing loss and tinnitus.

**Why the Correct Answer is Right**
The investigation of choice for diagnosing vestibular schwannoma is MRI (Magnetic Resonance Imaging) with gadolinium enhancement. This is because MRI provides high-resolution images of the internal auditory canal and the cerebellopontine angle, where the vestibular nerve is located. Gadolinium enhancement helps to visualize the tumor and its relationship with the surrounding neural structures. The sensitivity and specificity of MRI for detecting vestibular schwannoma are higher compared to CT (Computed Tomography) scans or other imaging modalities.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scans are less sensitive and less specific for detecting vestibular schwannoma compared to MRI.
**Option B:** Auditory evoked potentials (AEPs) are useful for assessing the function of the auditory nerve but are not diagnostic for vestibular schwannoma.
**Option C:** Electrophysiological studies such as electrocochleography may be helpful in assessing the function of the cochlea and auditory nerve, but are not the investigation of choice for diagnosing vestibular schwannoma.

**Clinical Pearl / High-Yield Fact**
MRI with gadolinium enhancement is the gold standard for diagnosing vestibular schwannoma, and it is essential to consider this diagnosis in patients presenting with unilateral hearing loss, tinnitus, and aural fullness.

**Correct Answer: C. MRI with gadolinium enhancement.**