**Core Concept**
Entrapment neuropathy refers to the compression of a peripheral nerve, leading to damage and disruption of nerve function. This condition often presents with pain, numbness, and weakness in the affected limb. Electrophysiological studies are essential for diagnosing entrapment neuropathy.

**Why the Correct Answer is Right**
The investigation of choice for entrapment neuropathy is nerve conduction studies (NCS) combined with electromyography (EMG). NCS measures the speed and strength of electrical signals traveling through nerves, while EMG assesses the electrical activity of muscles. By comparing the results, healthcare providers can identify areas of nerve compression and damage. This diagnostic approach is particularly useful for conditions such as carpal tunnel syndrome, ulnar nerve entrapment, and median nerve entrapment.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is useful for visualizing soft tissue structures and guiding injections, but it is not the primary investigation for entrapment neuropathy.
**Option B:** Magnetic Resonance Imaging (MRI) can provide detailed images of nerves and surrounding tissues, but it is not as sensitive as NCS and EMG for detecting nerve compression and damage.
**Option C:** Plain radiographs may show bony abnormalities that contribute to nerve compression, but they are not sufficient for diagnosing entrapment neuropathy.

**Clinical Pearl / High-Yield Fact**
When suspecting entrapment neuropathy, it's essential to perform a thorough clinical examination, including a detailed history and physical examination, to identify risk factors and potential causes of nerve compression.

**Correct Answer: C. MRI