**Question:** Simplest investigation to be performed in suspected cases of gastric cancer is -

A. Upper gastrointestinal (GI) endoscopy
B. Computed tomography (CT) scan
C. Abdominal ultrasound
D. Blood tests (e.g., CEA)

**Correct Answer:** A. Upper GI endoscopy

**Core Concept:**
Gastric cancer is a malignant tumor originating from the epithelial cells of the stomach. Upper gastrointestinal (GI) endoscopy is a non-invasive procedure that allows visualization and examination of the upper GI tract, including the esophagus, stomach, and duodenum. This is essential in suspected cases of gastric cancer as it provides direct visualization of the affected area, allowing for diagnosis, staging, and biopsy sampling.

**Why the Correct Answer is Right:**
A. Upper GI endoscopy is the correct answer because it directly visualizes the stomach and surrounding structures, enabling the detection of gastric cancer, as well as determining the extent of the tumor and involvement of adjacent organs.

**Why Each Wrong Option is Incorrect:**
B. Computed tomography (CT) scan is not the simplest investigation for gastric cancer diagnosis. Although CT scans can detect tumor involvement of adjacent organs and lymph node enlargement, they lack the capability to directly visualize the stomach and confirm the diagnosis of gastric cancer.
C. Abdominal ultrasound is not a suitable method for detecting gastric cancer. Ultrasonography primarily assesses the liver, pancreas, and gallbladder, and is not sensitive enough to detect early-stage gastric cancer.
D. Blood tests, such as CEA (Carcinoembryonic antigen), can be raised in gastric cancer patients but are not diagnostic. Blood tests are valuable in monitoring response to treatment but lack specificity and cannot confirm the diagnosis.

**Clinical Pearl:**
In suspected cases of gastric cancer, a combination of clinical history, physical examination, and the mentioned diagnostic modalities (endoscopy, CT scan, blood tests, and ultrasound) should be used to confirm and stage the disease. However, endoscopy remains the primary diagnostic tool for gastric cancer due to its direct visualization of the stomach and surrounding structures.