Which electrolyte is most critical to replace in a dehydrated 4-week-old infant with gastric outlet obstruction?

In the context of a dehydrated 4-week-old infant with gastric outlet obstruction, potassium is particularly critical to replace. Infants in this situation often experience significant losses of potassium through vomiting, which can disrupt their electrolyte balance. Potassium is essential for various physiological processes, including maintaining normal cellular function, muscle contraction, and cardiac health.

When gastric outlet obstruction occurs, the infant may not be able to retain nutrients and electrolytes, leading to dehydration and potential hypokalemia (low potassium levels). This can manifest as weakness, lethargy, and even cardiac arrhythmias, making it vital to closely monitor and replace potassium levels during treatment.

In contrast, other electrolytes like sodium, calcium, and magnesium are also important, but the immediate risk of serious complications from low potassium levels makes it the primary focus in cases of dehydration associated with gastric outlet obstruction in an infant. The rapid correction of potassium levels is crucial to prevent life-threatening conditions.