Question

A 5-month-old female infant presents with a 3-day history of vomiting. She is exclusively breastfed, and her mother states that today she has vomited within 15 minutes of each feeding. Her last wet diaper was 10 hours ago. On physical examination, she is afebrile, tachycardic, irritable, and does not express tears when crying.

She was a full-term infant born via vaginal delivery. She has no significant past medical history. Her 3-year-old sister has had gastroenteritis for the past few days.

What is the most likely laboratory finding?

1. Decreased blood urea nitrogen
2. Hyponatremia
3. Hypokalemia
4. Decreased urine specific gravity
5. Hypernatremia

Answer 1

The infant's symptoms strongly indicate dehydration due to vomiting, and the most likely laboratory finding is hyponatremia.

Step-by-step explanation:

The clinical presentation of a 5-month-old infant with a history of vomiting, irritability, and decreased tear production suggests that she is experiencing symptoms of dehydration, likely as a result of acute gastroenteritis, which could be suggested by her sister's similar symptoms. Given that she has not had a wet diaper in 10 hours, tachycardia, and her other symptoms, the most probable laboratory finding would be hyponatremia. This occurs due to the body losing more sodium-containing fluids than water, which is common in cases of frequent vomiting and inadequate fluid intake.