**Core Concept**
The urea breath test (UBT) is a diagnostic tool used to detect the presence of Helicobacter pylori (H. pylori) infection in the stomach. This test exploits the bacteria's ability to break down urea into carbon dioxide and ammonia.

**Why the Correct Answer is Right**
The UBT is based on the fact that H. pylori produces the enzyme urease, which catalyzes the hydrolysis of urea into carbon dioxide and ammonia. When a patient ingests a small amount of urea labeled with ¹⁴C or ¹³C, the bacteria in the stomach convert the urea into carbon dioxide, which is then exhaled and detected in the breath. This test is highly specific for H. pylori infection and is often used as a non-invasive alternative to endoscopy.

**Why Each Wrong Option is Incorrect**
**Option A:** The stool antigen test is a different diagnostic tool used to detect H. pylori infection, but it involves testing for the presence of antigens in stool samples, not urea breakdown products in breath.

**Option B:** The rapid urease test (RUT) is another diagnostic tool used to detect H. pylori infection, but it involves placing a biopsy sample from the stomach lining on a gel medium that contains urea, which turns pink if the sample is positive.

**Option C:** The endoscopy with biopsy is a diagnostic procedure used to visualize the stomach lining and collect tissue samples for histological examination, but it is not a test for urea breakdown.

**Clinical Pearl / High-Yield Fact**
The urea breath test is a useful diagnostic tool for H. pylori infection, but it is essential to consider the patient's recent use of antibiotics or proton pump inhibitors, as these can interfere with test results.

**Correct Answer: D. H. pylori infection**