Q. A 2-year-old male with Down Syndrome was admitted for vomiting. The patient’s parents report that he has been vomiting intermittently for the past 2 days. The vomiting is described as non-bilious and non-bloody. They report the patient's appetite has decreased and he has not been able to keep any food or liquids down. He has also had decreased urine output. The parents report that the patient has had no fever, diarrhea, or abdominal pain. They also report that the patient's behavior and mood have been normal. On examination, the patient appeared uncomfortable and had a distended abdomen. The abdomen was soft and nontender. The patient's vital signs were within normal limits, and he was admitted for further evaluation and management of his vomiting. The patient lives at home with his parents and two older siblings. The family lives in a single-family home in a low-income neighborhood. The patient's parents are both unemployed and have limited access to healthcare. The patient receives early intervention services for his developmental delay. The patient was born at 37 weeks via spontaneous vaginal delivery with a birth weight of 2.9 kg (6 lb 5 oz). He had no complications during the pregnancy or delivery. He was diagnosed with Down Syndrome at birth. He has had two previous hospitalizations for respiratory infections. He has no known allergies and is up to date on his vaccinations. An abdominal x-ray was performed on admission (see image). The patient's white blood cell count was elevated at 15,000/mm3. His serum sodium level was low at 130 mEq/L and his serum potassium level was also low at 3.5 mEq/L. Based on the information you have at this point in time, which of the following is the most likely diagnosis?