**Core Concept**
The urea breath test (UBT) is a non-invasive diagnostic tool used to detect the presence of active H. Pylori infection by measuring the amount of carbon dioxide produced when the bacteria break down urea in the stomach.

**Why the Correct Answer is Right**
The UBT works by having the patient ingest a small amount of radioactive or non-radioactive urea, which is then broken down by the H. Pylori bacteria into carbon dioxide. The patient then exhales, and the amount of carbon dioxide produced is measured using a breath analyzer. If the patient has an active H. Pylori infection, the urea will be broken down, and the carbon dioxide levels will be elevated. This test is particularly useful for detecting residual H. Pylori infection after treatment, as it can distinguish between active and past infections.

**Why Each Wrong Option is Incorrect**

**Option A:** The rapid urease test is a point-of-care test that detects the presence of H. Pylori by measuring the urease activity in a biopsy sample. While it can be used to diagnose H. Pylori infection, it is not the best method for detecting residual infection after treatment.

**Option C:** Endoscopy and biopsy are invasive procedures that can be used to diagnose H. Pylori infection by examining the stomach lining for signs of inflammation and collecting tissue samples for testing. However, they are not the best methods for detecting residual H. Pylori infection after treatment, as they are more invasive and may not be necessary for diagnosis.

**Option D:** Serum anti-H. Pylori titre is a blood test that detects the presence of antibodies against H. Pylori. However, this test can remain positive for several months after treatment, making it less useful for detecting residual infection.

**Clinical Pearl / High-Yield Fact**
The urea breath test is a non-invasive, cost-effective, and accurate method for detecting H. Pylori infection, making it an excellent choice for post-treatment surveillance.

**✓ Correct Answer: B. Urea breath test**