You are a third year medical student completing your first rotation in ambulatory clinic in a general pediatrics clinic. Your preceptor sends you in to see a mother with a child complaining of vomiting.

A mother has brought in her four-week old baby boy for complaints of vomiting. She explains that over the past couple days, the child has been throwing up, and more recently the vomit nearly “hit the wall.” After a few minutes, she continues feeding but the same series of events occurred. This morning, five more attempts of feeding were followed by five episodes of projectile vomiting. As this is her first child, she is obviously quite concerned and came to the office.

You attempt to characterize the color of the vomiting and the mother describes it as milky. Upon further history taking, the mother notes that the pregnancy and delivery were both uncomplicated. There is no family history of any medical conditions of note. Otherwise, the rest of the history is unremarkable.

General appearance: Overall well looking 4-week old baby boy, easily distractible
Vitals: HR: 135 beats/min, BP: 70/46, RR: 35/min, T: 37.0 C
HEENT: Slightly parched mucous membranes, fontanelle slightly sunken
CVS: Split S1, S2, II/VI systolic ejection murmur
Pulmonary: Chest clear to auscultation
Abdo: Soft and nontender.
Neuro: Screening exam normal.
Skin: skin turgor slightly decreased

Before you return to your preceptor to present the finds, what are your top three differential diagnoses?