Pyloric stenosis
Progressive hypertrophy of the pyloric muscle narrows the gastric outlet until only forceful contraction can empty the stomach, so the vomiting is projectile and non-bile-stained, and the electrolyte picture is a direct consequence of losing acidic gastric contents rather than the obstruction itself.
First principles
The problem is progressive muscular narrowing of a single fixed point
In pyloric stenosis, the circular smooth muscle of the pylorus hypertrophies and hyperplasias for reasons that are not fully understood, gradually narrowing the gastric outlet over the first few weeks of life. Because the pylorus is the single channel through which everything the stomach contains must pass, this is an obstruction with no alternative route, unlike more diffuse gut pathology: the entire clinical picture follows from an outlet that becomes progressively harder, and eventually impossible, to push milk through.
Educational content pending clinical review. Not medical advice.