**Core Concept**
Gastric outlet obstruction (GOO) in infants often results from congenital anomalies, infections, or acquired conditions. In a 4-week-old baby, the most common cause is related to the development of the gastrointestinal tract.

**Why the Correct Answer is Right**
The most common cause of gastric outlet obstruction in a 4-week-old baby is **Duodenal Atresia**. Duodenal atresia is a congenital anomaly where the duodenum (the first part of the small intestine) is partially or completely blocked. This blockage prevents food from passing through the digestive system, leading to gastric outlet obstruction. The obstruction is usually due to a failure of recanalization of the duodenum during fetal development.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pyloric stenosis is a condition where the pylorus, the passage between the stomach and the small intestine, becomes narrowed. However, it typically presents later in infancy, around 3-6 weeks of age, and is less common than duodenal atresia.
* **Option B:** Intestinal malrotation is a condition where the intestine does not develop properly, leading to a twisted or abnormal position. While it can cause obstruction, it is less common than duodenal atresia in this age group.
* **Option C:** Gastroesophageal reflux disease (GERD) is a condition where stomach acid flows back into the esophagus, causing discomfort. However, it is not typically associated with gastric outlet obstruction.

**Clinical Pearl / High-Yield Fact**
In infants, gastric outlet obstruction often presents with projectile vomiting, abdominal distension, and failure to thrive. The "double bubble" sign on abdominal radiographs is a classic finding in duodenal atresia.

**Correct Answer:** C.