A patient passes very large volumes of dilute urine but the urine contains no glucose. The most likely condition is

Explanation

Diabetes insipidus results from a lack of ADH or a failure of the tubule to respond to it, so water cannot be reabsorbed and the patient produces litres of dilute, sugar free urine and is constantly thirsty. Diabetes mellitus also causes a large urine volume, but there the cause is unreabsorbed glucose holding water in the tubule, so glucose is present. The word diabetes simply means a large flow.