**Question:** Which of the following investigations work on the same principle?

A. Electrocardiography (ECG)
B. Electroencephalography (EEG)
C. Ultrasonography
D. Magnetic Resonance Imaging (MRI)

**Core Concept:** The question is testing the understanding of the fundamental principle behind the electrical recording techniques used in medical investigations. These techniques measure electrical signals generated by specific cells or tissues and are used to assess various physiological functions or pathophysiological processes.

**Why the Correct Answer is Right:** Electrocardiography (ECG) measures the electrical activity of the heart, reflecting the depolarization and repolarization of cardiac cells during each heartbeat. Electroencephalography (EEG) records the electrical activity of the brain, reflecting the electrical activity of neurons. Both ECG and EEG are used to assess cardiac and brain functions, respectively.

**Why Each Wrong Option is Incorrect:**

Option A: Ultrasonography (USG) is a diagnostic imaging technique that uses high-frequency sound waves to create images of internal body structures. It works by reflecting and transmitting sound waves, and is not based on electrical signal measurement.

Option D: Magnetic Resonance Imaging (MRI) is a diagnostic imaging technique that uses strong magnetic fields, radiofrequency pulses, and a computer to generate detailed images of internal body structures. It is a different method from ECG, EEG, and does not involve electrical signal measurement.

**Clinical Pearl:** The correct answer demonstrates the understanding of the principles behind ECG and EEG, which are essential for assessing cardiac and brain functions, respectively. These techniques provide valuable information for diagnosing various cardiac and neurological disorders. Remember that other imaging modalities like USG and MRI use entirely different principles for imaging internal body structures.