**Core Concept:** Peripheral neuropathy (PN) is a disorder affecting the peripheral nerves, which are responsible for transmitting sensory, motor, and autonomic functions from the central nervous system to the rest of the body. The correct investigation for PN depends on the suspected etiology (e.g., vitamin B12 deficiency, diabetes, autoimmune, toxic, or genetic causes).

**Why the Correct Answer is Right:**

The correct answer is **D.** Magnetic Resonance Imaging (MRI), as it is an advanced imaging technique that can detect morphological changes in peripheral nerves, nerve roots, and entrains. MRI is particularly useful in evaluating the nerves and their surrounding structures, which can help identify the cause of PN. MRI is not limited by signal alterations due to edema, demyelination, or axonal degeneration, which are common findings in various forms of PN.

**Why Each Wrong Option is Incorrect:**

A. **Ultrasonography (USG):** Ultrasound is a limited imaging modality for evaluating peripheral nerves due to its inability to penetrate soft tissues and visualize the nerves' morphology. It is more suitable for evaluating bony structures, joints, and blood vessels.

B. **Electromyography (EMG) and Nerve Conduction Studies (NCS):** EMG and NCS are electrophysiological tests used to evaluate the function of peripheral nerves and motor/sensory neurons. While these tests can help assess the function of peripheral nerves, they cannot provide morphological information and are not suitable for evaluating the cause of peripheral neuropathy.

C. **Blood tests:** Blood tests are essential for evaluating vitamin B12 deficiency, but they are not a substitute for imaging studies to visualize the peripheral nerves and identify morphological changes.

**Clinical Pearl:** A comprehensive approach to diagnosing peripheral neuropathy involves a combination of clinical examination, blood tests, and appropriate imaging modality (MRI in this case), depending on the suspected etiology, to accurately diagnose and guide management.