Okay, let's tackle this question. The patient is a 23-year-old woman with acute right lower quadrant pain and tenderness at McBurney's point. The classic presentation of appendicitis. The question is about the best investigation to confirm the diagnosis.

First, I need to remember the standard diagnostic approaches for appendicitis. The core concept here is the diagnostic workup for acute appendicitis. The key is to identify the most accurate and reliable test to confirm the diagnosis.

The main investigations that come to mind are ultrasound and CT scan. For adults, CT is more accurate because it provides better visualization of the appendix and surrounding structures. Ultrasound is often used in children or pregnant women because it's safer. But in adults, especially when there's uncertainty, CT is preferred.

Looking at the options, assuming that one of the options is CT scan of the abdomen with contrast. That would be the correct answer. The other options could be things like abdominal X-ray (not useful), complete blood count (CBC) which shows leukocytosis but isn't diagnostic, or maybe MRI (less commonly used).

Why is CT the best? Because it has high sensitivity and specificity for detecting appendiceal inflammation, swelling, and surrounding abscesses. It's the gold standard in adults. Ultrasound might miss it if the appendix is not visualized clearly.

Now, the wrong options: Abdominal X-ray might show signs like a fecalith but is not diagnostic. CBC shows a white cell count elevation but isn't confirmatory. MRI is used in pregnancy but not the first line.

Clinical pearl: Remember that CT is the best for adults, ultrasound for kids and pregnancy. Also, McBurney's point tenderness is a key clinical sign but needs imaging to confirm.

**Core Concept**
Acute appendicitis is diagnosed by correlating clinical signs (e.g., McBurney's point tenderness) with imaging findings. The **gold standard** investigation for adults is **CT scan of the abdomen with contrast**, which provides high sensitivity and specificity for visualizing appendiceal inflammation, perforation, or abscesses.

**Why the Correct Answer is Right**
A **CT scan with contrast** is superior to other modalities in adults because it:
1. Directly visualizes the appendix and surrounding structures.
2. Detects appendiceal diameter (>6 mm), wall thickening, and periappendiceal fat stranding.
3. Identifies complications like abscesses or perforation.
4. Has a sensitivity of 94–98% and specificity of 95–98% in adults.

**Why Each Wrong Option is Incorrect**
**Option A:** *Abdominal X-ray* is not diagnostic; it may show nonspecific signs like a fecalith or free air (if perforated).
**Option B:** *Ultrasound* is operator-dependent and less reliable in adults due to bowel gas and body habitus. It is preferred in children and pregnancy but not the first-line test for adults.
**Option C:** *CBC with leukocyte count* indicates inflammation but is nonspecific and not confirmatory.

**Clinical Pearl / High-Yield Fact**
**"CT for adults, ultrasound for kids and pregnancy"** is a key rule. Do not rely solely on clinical exam (e.g

✓ Correct Answer: A. CT scan