## **Core Concept**
Acute haematemesis refers to the vomiting of blood, which is a symptom that requires immediate medical attention. It is often a sign of upper gastrointestinal bleeding. Diagnostic procedures for acute haematemesis aim to identify the source and cause of the bleeding.

## **Why the Correct Answer is Right**
In the context of acute haematemesis, an **endoscopy** (option not listed but implied as a comparison) is considered the gold standard for diagnosing the cause of upper gastrointestinal bleeding. Among the given options, **. Upper GI endoscopy** is directly related to a useful diagnostic procedure. However, **. Ultrasound** (implied as a less direct method) might not directly visualize the source of bleeding within the GI tract but can help in assessing liver conditions or portal hypertension. Given the context and common practices, **. Ba swallow** (Barium swallow) might not be as immediately useful in acute settings due to its lower sensitivity for active bleeding and the difficulty in performing it on an unstable patient.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Not provided, assuming relates to a useful diagnostic tool.
- **Option B:** Assuming **B. Ultrasound**, while not the best for direct visualization of GI mucosa, it has utility in assessing liver disease or portal hypertension which could cause haematemesis. However, it's not entirely less useful but context-dependent.
- **Option C:** Not provided.
- **Option D:** Not provided.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in cases of acute haematemesis, **upper GI endoscopy** within 24 hours of presentation is recommended for patients who are hemodynamically unstable or have signs of severe bleeding. This procedure allows for direct visualization of the bleeding site and potential therapeutic interventions.

## **Correct Answer: B. Ultrasound.**