**Core Concept**
The clinical presentation of non-bilious projectile vomiting in an infant suggests a condition affecting the upper gastrointestinal tract, likely **pyloric stenosis**. This condition is characterized by the thickening of the **pylorus muscle**, leading to gastric outlet obstruction. The investigation of choice should be able to demonstrate the anatomical abnormality.

**Why the Correct Answer is Right**
A **barium meal** is the correct investigation of choice because it can clearly show the narrowed pyloric channel and the characteristic "string sign" or "railroad track sign" due to the passage of barium through the narrowed pylorus. This diagnostic tool is valuable in visualizing the upper gastrointestinal tract and can help confirm the diagnosis of **pyloric stenosis** by demonstrating the obstruction.

**Why Each Wrong Option is Incorrect**
**Option B:** Abdominal ultrasonography, while useful in many pediatric abdominal conditions, is not the primary investigation for suspected pyloric stenosis, although it can be used to measure the thickness of the pyloric muscle.
**Option C:** Flexible Endoscopy is more invasive and not typically used for the initial diagnosis of pyloric stenosis in infants.
**Option D:** Sigmoidoscopy is used to examine the lower gastrointestinal tract and is not relevant to the diagnosis of conditions causing upper gastrointestinal symptoms like pyloric stenosis.

**Clinical Pearl / High-Yield Fact**
In infants presenting with non-bilious projectile vomiting, it's crucial to consider **pyloric stenosis** and to perform a **barium meal** as the investigation of choice. Early diagnosis is key to preventing complications such as dehydration and electrolyte imbalance.

**✓ Correct Answer: A. Barium meal**