I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
A 26-year-old man comes to the office because of a 1-week history of increased urinary frequency accompanied by excessive thirst. He says he has been urinating hourly. Physical examination shows no abnormalities. Serum chemistry studies are within the reference ranges. Urine osmolality is 50 mOsmol/kg H2O. After administration of ADH (vasopressin), his urine osmolality is within the reference range. The most likely cause of this patient’s symptoms is dysfunction of which of the following structures?
(A) Anterior pituitary gland
(B) Bowman capsule
(C) Glomerulus
(D) Hypophysial portal system
(E) Loop of Henle
(F) Supraoptic nucleus
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Answer
The patient’s symptoms of increased urinary frequency and excessive thirst suggest diabetes insipidus (DI), a condition characterized by the inability to concentrate urine, leading to large volumes of dilute urine. The urine osmolality is very low (50 mOsmol/kg H2O), indicating dilute urine.
The fact that administration of ADH (vasopressin) normalizes the urine osmolality suggests that the patient has central diabetes insipidus (CDI), which occurs due to a deficiency in the production or release of ADH. ADH is produced in the supraoptic nucleus of the hypothalamus and is released from the posterior pituitary gland.
The supraoptic nucleus dysfunction results in insufficient ADH production, leading to the inability of the kidneys to concentrate urine, causing symptoms like polyuria and polydipsia.
The correct answer is F