**Core Concept:** Appendicitis is an inflammation of the appendix, a common cause of acute abdominal pain, especially in young adults. McBurney's point is a specific anatomic landmark for identifying the site of pain in appendicitis.

**Why the Correct Answer is Right:** Among the given options, abdominal ultrasound (USG) is the best choice for confirming appendicitis due to its non-invasive nature, availability, and ability to visualize the appendix. Ultrasound can demonstrate features such as appendiceal wall thickening, fluid collections, and air within the appendix (gas-filled appendix), which are specific for appendicitis.

**Why Each Wrong Option is Incorrect:**

A. Radionuclide scan (option A): This is a radioisotope scan and is not commonly used for diagnosing appendicitis due to its invasiveness, limited availability, and poor sensitivity for acute appendicitis.

B. Computed tomography (CT) scan (option B): While CT is a sensitive tool for diagnosing appendicitis, it involves radiation exposure, making it less appealing compared to ultrasound. Additionally, CT scans are not always immediately available in emergency situations.

C. Colonoscopy (option C): Colonoscopy is an invasive procedure specifically for examining the colon, not the appendix. It is not suitable for the diagnosis of appendicitis.

D. Abdominal X-ray (option D): An abdominal X-ray is not sensitive enough to detect appendicitis, especially in early stages. Appendicitis may present with bowel gas patterns, but this is not specific for appendicitis and can be seen in other conditions as well.

**Clinical Pearls:** Ultrasound is a reliable non-invasive investigation for diagnosing appendicitis, making it an ideal choice for emergency departments and clinical settings. In cases where ultrasound is not available or inconclusive, a CT scan can be considered as an alternative, albeit with radiation exposure and limited availability.