The nurse is caring for a 3-year-old with persistent diarrhea, subject to significant fluid and electrolyte alterations.

Choice A rationale Persistent diarrhea leads to significant loss of bicarbonate (HCO3−) from the gastrointestinal tract, as the lower GI contents are highly alkaline. This loss of base results in an excess relative concentration of hydrogen ions (H+) in the extracellular fluid, causing a decrease in blood pH (normal pH 7.35 to 7.45) and a reduction in serum bicarbonate (normal HCO3− 22 to 26 mEq/L), which is the definition of metabolic acidosis. Choice B rationale Respiratory acidosis is caused by hypoventilation, leading to retention of carbon dioxide (CO_2), which then combines with water to form carbonic acid. Diarrhea does not directly impair respiratory function; in fact, the body compensates for metabolic acidosis by increasing ventilation (Kussmaul breathing) to blow off CO_2. Choice C rationale Metabolic alkalosis is characterized by an excessive loss of acid or an excessive retention of base, resulting in an elevated blood pH and an elevated serum bicarbonate level. This is commonly seen with prolonged vomiting (loss of gastric HCl) or diuretic use, which is the opposite of the fluid and electrolyte loss pattern seen with diarrhea. Choice D rationale Respiratory alkalosis is caused by hyperventilation, leading to an excessive loss of carbon dioxide (CO_2), resulting in a decrease in the partial pressure of CO_2 (PaCO_2 normal 35 to 45 mmHg) and an elevated blood pH. While the child may hyperventilate as a compensatory mechanism, this is not the primary physiological imbalance caused by diarrhea itself.