**Core Concept**
The patient's presentation of non-bilious vomiting in a young infant suggests an upper gastrointestinal obstruction, specifically pyloric stenosis. This condition 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 shows thickening of the pyloric muscle (typically >4 mm) and elongation of the pyloric channel. The correct next step in management is to perform a pyloromyotomy, a surgical procedure that involves cutting the hypertrophied muscle to relieve the obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper GI series with barium swallow is not the next step in management, as it is more informative for lower GI obstructions or malrotation. While it may show the pyloric channel, it is not necessary for diagnosis in this case.
**Option B:** Endoscopy is not the next step in management, as it is more useful for diagnosing mucosal lesions or foreign bodies in the upper GI tract. It may not provide useful information in this case.
**Option C:** Nasogastric tube decompression may provide temporary relief but does not address the underlying cause of the obstruction. It is not a definitive treatment for pyloric stenosis.

**Clinical Pearl / High-Yield Fact**
In infants with pyloric stenosis, the "olive" sign, which refers to a palpable mass in the epigastrium, is a classic physical examination finding. However, it is not always present, and ultrasound is the preferred diagnostic method.

**Correct Answer:** C.