**Core Concept**
An acoustic neuroma, also known as a vestibular schwannoma, is a benign tumor that arises from the Schwann cells of the vestibular nerve, leading to symptoms such as hearing loss, tinnitus, and balance disturbances. The ideal diagnostic investigation should be sensitive and specific for detecting this tumor.

**Why the Correct Answer is Right**
Magnetic Resonance Imaging (MRI) with gadolinium enhancement is the preferred imaging modality for diagnosing acoustic neuromas. This is because MRI provides excellent soft tissue contrast, allowing for the detection of small tumors that may not be visible on computed tomography (CT) scans. The addition of gadolinium enhancement helps to highlight the tumor's vascular characteristics, making it easier to distinguish from surrounding structures.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scans may be used as a preliminary investigation, but they are less sensitive and specific than MRI for detecting acoustic neuromas.
**Option B:** Auditory brainstem response (ABR) testing can help identify hearing loss and balance disturbances, but it is not a direct imaging modality for detecting tumors.
**Option C:** Electrophysiological studies, such as electromyography (EMG) and electroencephalography (EEG), are not relevant for diagnosing acoustic neuromas.

**Clinical Pearl / High-Yield Fact**
When interpreting MRI scans for acoustic neuromas, it's essential to look for the "dumb-bell" sign, where the tumor causes a characteristic expansion of the internal auditory canal.

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