UKMLA PLAB AMC USMLE Prometric MCQ

Correct Answer: B

Explanation:
This patient probably has nephrogenic diabetes insipidus secondary to lithium therapy. Diabetes Insipidus is associated with polyuria and polydipsia. Urine with a low specific gravity and blood with an elevated serum osmolality is consistent with diabetes insipidus.A patient with uncontrolled diabetes mellitus (choice a) would also have polydipsia and polyuria. However, there would be glucose in his urine and his blood glucose level would be elevated.A person with psychogenic polydipsia (choice c) also would drink large amounts of water and have a large urine output. In psychogenic polydipsia, the blood osmolality would be lower than normal, not greater than normal, as in this case.A patient with the syndrome of inappropriate ADH secretion (choice d) would have serum hypo-osmolality and serum hyponatremia. The urine would be hyper osmotic.Oliguria (choice e) is scanty urine output. This is not the case in this scenario.