Final answer:
An infant with severe isotonic dehydration is at the highest risk for hypovolemic shock.
Step-by-step explanation:
An infant with severe isotonic dehydration is at the highest risk for hypovolemic shock. Severe isotonic dehydration refers to a condition where there is a loss of water and electrolytes without a significant change in the concentration of solutes. This can lead to a decrease in blood volume and circulation failure, which is characteristic of hypovolemic shock.