**Core Concept**
Congenital hypertrophic pyloric stenosis is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction in infants. This results in projectile vomiting, dehydration, and electrolyte imbalances.

**Why the Correct Answer is Right**
The correct answer, hypochloremic alkalosis, is a direct consequence of persistent vomiting in congenital hypertrophic pyloric stenosis. Vomiting leads to the loss of hydrogen ions (H+) and chloride ions (Cl-) from the stomach, resulting in a decrease in chloride levels (hypochloremia) and an increase in bicarbonate levels (alkalosis). This condition is often associated with metabolic alkalosis, which can be severe and life-threatening if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** More common in girls. While the exact incidence of congenital hypertrophic pyloric stenosis is not significantly different between boys and girls, it is more common in firstborn males.

**Option C:** Heller's myotomy is the procedure of choice. Heller's myotomy is a surgical procedure typically used to treat achalasia, not congenital hypertrophic pyloric stenosis. The procedure of choice for congenital hypertrophic pyloric stenosis is pyloromyotomy, which involves making a longitudinal incision in the pyloric muscle to relieve the obstruction.

**Option D:** Most often manifests at birth. While congenital hypertrophic pyloric stenosis can present at birth, it typically becomes apparent within the first few weeks of life, with a peak incidence around 3-5 weeks of age.

**Clinical Pearl / High-Yield Fact**
A classic clinical sign of congenital hypertrophic pyloric stenosis is the "olive" mass, which is a palpable, mobile mass in the epigastric region, representing the hypertrophied pyloric muscle. This sign is often associated with projectile vomiting and is a key diagnostic feature of the condition.

**✓ Correct Answer: B. Hypochloremic alkalosis**