**Core Concept**
Gastrointestinal obstruction in infants often presents with nonbilious vomiting, constipation, and weight loss. The most common cause of intestinal obstruction in neonates is a congenital anomaly, such as intestinal atresia or stenosis.

**Why the Correct Answer is Right**
The diagnosis of intestinal obstruction in a 26-day-old infant with recurrent nonbilious vomiting, constipation, and weight loss is likely intestinal atresia. This condition occurs when a portion of the intestine is missing or narrowed, preventing the normal flow of intestinal contents. The vomiting is nonbilious because the obstruction is distal to the ampulla of Vater, which marks the entry point of the bile duct into the small intestine. The constipation is due to a decreased intestinal motility and a decrease in the intestinal secretions. The weight loss is a result of the inability to absorb nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:** pyloric stenosis typically presents with projectile nonbilious vomiting in infants, but it usually occurs after the first few weeks of life. The diagnosis of pyloric stenosis is less likely in a 26-day-old infant with constipation.
**Option B:** gastroesophageal reflux disease (GERD) presents with frequent regurgitation of milk and vomiting in infants, but it is not typically associated with constipation or weight loss.
**Option C:** intestinal perforation presents with sudden onset of abdominal distension, abdominal pain, and vomiting, but it is not typically associated with constipation or weight loss in the early stages.
**Option D:** Hirschsprung's disease presents with constipation, abdominal distension, and failure to pass meconium in the first 48 hours of life, but it is not typically associated with nonbilious vomiting.

**Clinical Pearl / High-Yield Fact**
In infants presenting with nonbilious vomiting, constipation, and weight loss, a high index of suspicion for intestinal atresia or stenosis is essential. A plain abdominal radiograph may show a characteristic "double bubble" sign, which is indicative of duodenal atresia.

**Correct Answer:** C. Intestinal atresia.