**Core Concept**
Congenital hypertrophic pyloric stenosis (CHPS) is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction in infants. This condition is caused by an abnormal increase in the smooth muscle mass of the pylorus, resulting in narrowing of the pyloric channel.

**Why the Correct Answer is Right**
The correct answer is **A. Proximal gastric dilatation**. In CHPS, the thickened pyloric muscle causes gastric outflow obstruction, leading to accumulation of food and fluid in the stomach. This results in proximal gastric dilatation, which is a hallmark feature of the condition. As the stomach distends, it can lead to nausea, vomiting, and dehydration in infants.

**Why Each Wrong Option is Incorrect**
**Option B:** **Heterochromia** is a condition characterized by abnormal pigmentation of the iris, and it is not associated with CHPS.

**Option C:** **Duodenal atresia** is a congenital anomaly of the small intestine, which can also present with vomiting, but it is not characterized by proximal gastric dilatation.

**Option D:** **Meckel's diverticulum** is a congenital anomaly of the small intestine, which can present with gastrointestinal bleeding, but it is not associated with CHPS.

**Clinical Pearl / High-Yield Fact**
CHPS typically presents in infants between 2-8 weeks of age, with a peak incidence at 3-5 weeks. It is more common in male infants and is often associated with a family history.

**Correct Answer: A. Proximal gastric dilatation**