**Question:** Investigation of choice for diagnosis of CA colon

**Core Concept:** Colorectal cancer (CRC) is a common malignancy worldwide, with a variety of screening methods used to detect early-stage cancer or precancerous lesions in the colon and rectum. The most effective method for diagnosis is a combination of stool tests and colonoscopy.

**Why the Correct Answer is Right:**

The correct answer is **D:** Colonoscopy. Colonoscopy is a highly sensitive and specific diagnostic tool for detecting CRC. It involves the insertion of a flexible video camera into the colon through the rectum, allowing visualization and biopsy of suspicious lesions. Colonoscopy is essential for obtaining tissue samples (biopsies) for histopathological examination to confirm the diagnosis of CRC.

**Why Each Wrong Option is Incorrect:**

A. Colon X-ray (Barium enema): This technique is less sensitive and specific compared to colonoscopy, as it does not allow direct visualization of the entire colon.

B. Stool tests (Fecal occult blood test, FOBT): These tests detect blood in the stool, which may indicate CRC or other colonic diseases. However, they are less sensitive in detecting early-stage CRC and have a high false-negative rate.

C. Abdominal ultrasound: Ultrasound is not suitable for diagnosing CRC, as it lacks the depth penetration and spatial resolution required to visualize the large bowel.

**Clinical Pearl:**

A combination of stool tests (e.g., FOBT) and colonoscopy is the gold standard for CRC screening, as it improves sensitivity and specificity. Colonoscopy allows direct visualization of the colon, enabling the detection of polyps and cancerous lesions, while stool tests detect blood in the stool.

**Correct Answer:** Colonoscopy (D) is the correct choice for diagnosis of CRC due to its superior sensitivity and specificity compared to other methods. However, a combination of stool tests and colonoscopy is recommended for optimal screening and diagnosis of CRC.