Increased hematocrit in this patient reflects hemoconcentration caused by dehydration, secondary to diarrhea. This hematologic condition, termed relative polycythemia, is characterized by decreased plasma volume with a normal red cell mass. When patients suffer from burns, vomiting, excessive sweating, or diarrhea, they not only lose fluid but also suffer electrolyte disturbances. Systemic blood pressure falls with continuous dehydration, and declining perfusion eventually leads to death. Diabetes insipidus (choice C) may cause dehydration but is an unlikely choice because the patient has a history of diarrhea. None of the other choices cause relative polycythemia.Diagnosis: Dehydration, relative polycythemia