**Core Concept**
The child's presentation of nonbilious vomiting and failure to gain weight, along with a palpable epigastric lump, suggests intestinal obstruction, likely due to a congenital anomaly such as pyloric stenosis. Pyloric stenosis is characterized by hypertrophy of the pyloric muscle, leading to narrowing of the pyloric channel and obstruction of gastric outflow.

**Why the Correct Answer is Right**
The diagnosis of pyloric stenosis is often confirmed by ultrasound, which can visualize the hypertrophied pyloric muscle and assess the pyloric channel length. Ultrasound is a non-invasive and quick method for diagnosing pyloric stenosis, especially in infants. It can help differentiate pyloric stenosis from other causes of vomiting and abdominal distension.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray of the abdomen may show a "double bubble" sign in cases of duodenal atresia but is not the first line of investigation for suspected pyloric stenosis.
**Option B:** Upper GI endoscopy is not typically used for the initial evaluation of pyloric stenosis.
**Option C:** CT scan is not necessary for the initial diagnosis of pyloric stenosis and carries a higher radiation burden.

**Clinical Pearl / High-Yield Fact**
In cases of pyloric stenosis, the "olive" sign can be elicited by palpation of the abdomen, where a firm, mobile mass is felt in the epigastric region.

**Correct Answer: C. CT scan is not necessary for the initial diagnosis of pyloric stenosis and carries a higher radiation burden.