A 55-year-old man presents to the hospital with flu-like sym…

A 55-year-old man presents to the hospital with flu-like symptoms, including nausea and vomiting, that began 4 days ago. He is tachycardic and has decreased skin turgor on exam. Lab results consist of a serum glucose level of 800 mg/dL, serum osmolality of 380 mOsm/kg, negative for ketones, and a normal anion gap. Which of the following conditions is the most likely cause of his dehydration?