Gastroenteritis in a child commonly causes which type of dehydration?

• A. Isotonic dehydration
• B. Hypotonic dehydration
• C. Hypertonic dehydration
• D. Dehydration with electrolyte imbalance

Answer: (A)

The key idea here is that dehydration type is defined by the balance of water and electrolyte losses and the resulting serum osmolality. In gastroenteritis, vomiting and, especially, diarrhea cause loss of fluid that closely matches the composition of body fluids—that is, isotonic fluid. When you lose isotonic fluid, you lose water and electrolytes in roughly equal amounts, so the serum osmolality stays normal. This leads to isotonic dehydration, which is the most common pattern seen in children with acute gastroenteritis.

That’s why isotonic dehydration best fits this scenario: the loss from the gut mirrors plasma in concentration, causing a reduced extracellular fluid volume with normal serum sodium, rather than a net shift toward either higher or lower osmolality.

Hypertonic dehydration would come from losing more water than electrolytes (raising the serum sodium), which isn’t typical with ordinary diarrhea. Hypotonic dehydration would result from losing more electrolytes than water (lowering the serum sodium). Dehydration described simply as having an electrolyte imbalance is less specific and doesn’t capture the usual proportional loss seen with gastroenteritis.