**Core Concept**
The clinical presentation of a 9-month-old girl with a palpable mass, masked liver dullness, and shock-like condition is suggestive of an abdominal catastrophe, likely an intussusception. Intussusception is a condition where a part of the intestine invaginates into another, leading to bowel obstruction, ischemia, and potential perforation.

**Why the Correct Answer is Right**
Intussusception is a pediatric emergency requiring prompt diagnosis and treatment. A barium enema is contraindicated in suspected intussusception as it can lead to perforation of the bowel, which can be fatal. The use of barium can make subsequent surgery more complicated and is not recommended as a diagnostic or therapeutic tool in this scenario.

**Why Each Wrong Option is Incorrect**
**Option B:** IV fluid is an essential component of the management of a child in shock, and it should be administered promptly to maintain circulation and perfusion of vital organs.
**Option C:** Oxygen supplementation is crucial in maintaining adequate oxygenation and supporting the child's respiratory needs, especially in a shock-like condition.
**Option D:** Nasogastric tube placement is necessary to decompress the stomach and relieve vomiting, which can help in reducing the risk of aspiration and improving the child's overall condition.

**Clinical Pearl / High-Yield Fact**
In a child with suspected intussusception, a plain abdominal X-ray may show a target sign or a sausage-shaped mass, which can aid in the diagnosis. However, these findings are not diagnostic, and a contrast enema or surgical exploration is often required for definitive diagnosis and treatment.

**✓ Correct Answer: A. Barium enema**