In an infant with mild dehydration from diarrhea, what is the most likely cause?

• A. A bacterial gastroenteritis
• B. Constipation
• C. An allergic reaction to formula
• D. A viral gastroenteritis that resolves in 72-96 hours

Answer: (D)

Most cases of diarrhea with mild dehydration in infants come from a viral gastroenteritis, which tends to be self-limiting. Viruses such as rotavirus or norovirus cause acute, watery diarrhea that typically lasts a few days, usually around 72–96 hours. This course fits the common pattern of a short, self-resolving illness in otherwise healthy infants. Bacterial gastroenteritis often brings higher fever, more severe dehydration, abdominal tenderness, or even blood in the stool, and tends to be less characteristic of a straightforward mild dehydration picture. Constipation does not cause diarrhea, and formula-related diarrhea can occur but is not the most likely, especially when the presentation is a brief, self-limited viral illness.