A 53-year-old man comes to the physician complaining of increased urinary frequency for the past 4 weeks. He has no burning with urination, urgency, or difficulty initiating urination. He wakes up several times at night to urinate. His mouth feels dry all the time and he drinks fluids almost every hour to alleviate his thirst. The patient's medications include hydrochlorothiazide and amlodipine for hypertension. His sister was diagnosed with diabetes mellitus at a young age. His blood pressure is 120/76 mm Hg, pulse is 85/min, and respirations are 15/min. He is afebrile. His body mass index is 32 kg/m². Laboratory results are as follows: Which of the following is most consistent with this patient's findings?

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11ec25c8_d3a7_0d1b_bc81_fb35d7a65636_MD0003_00 This patient has polyuria with dilute urine and increased water intake. Polyuria in the nonhospitalized patient is most commonly due to diabetes mellitus, primary polydipsia (previously known as psychogenic polydipsia), and central or nephrogenic diabetes insipidus (DI). This patient's normal glucose makes diabetes mellitus less likely. Primary polydipsia is due to increased water intake that leads to a dilute urine (urine osmolality < 1/2 plasma osmolality) and hyponatremia (sodium <137 mEq/L). Serum Na >145 mEq/L with dilute urine excludes primary polydipsia and suggests DI in this patient (Choice E). DI can be central or nephrogenic. Central DI is due to decreased antidiuretic hormone (ADH) secretion from the pituitary. Nephrogenic DI has normal ADH levels with varying degrees of renal ADH resistance. The decreased ADH action in both types of DI leads to decreased renal water reabsorption and water loss with polyuria. This results in dilute urine and hypernatremia. Central DI usually has significant hypernatremia (>150 mEq/L) due to an impaired thirst mechanism. Patients with nephrogenic DI usually have an intact thirst mechanism and adequate water intake; they usually compensate for renal water loss and may have a normal sodium level. (Choice A) Benign prostatic hyperplasia can present with urinary hesitancy, weak urinary stream, nocturia, and incomplete voiding with overflow incontinence. However, it does not usually cause this degree of dilute urine or hypernatremia. (Choices C and H) Osmotic diuresis is usually due to solutes (eg, glucose, mannitol, urea) that are not well reabsorbed in the kidney. Patients develop polyuria with a high serum osmolality. However, this patient's normal serum glucose and relatively normal blood urea nitrogen make this less likely. (Choice D) Hydrochlorothiazide (HCTZ) acts in the distal kidney and can cause hypokalemia, hyperglycemia, and hyperuricemia. Although this patient's polyuria could be due to HCTZ, diuretics more commonly cause hyponatremia instead of hypernatremia. In addition, diuretic use usually stimulates ADH release to result in concentrated urine rather than the very dilute urine seen in this patient. (Choice F) Renal tubular acidosis (RTA) usually presents with a normal anion gap metabolic acidosis. The serum bicarbonate is typically <20 mEq/L, which is not present in this patient. In addition, RTA does not typically cause the degree of hypernatremia or dilute urine seen in this patient. (Choice G) Syndrome of inappropriate antidiuretic hormone (SIADH) leads to inappropriately high ADH concentration and concentrated urine with urine osmolality > serum osmolality. The excess water reabsorption in the kidney leads to hyponatremia and a lower serum uric acid level. However, polyuria is not commonly associated with SIADH. This patient's dilute urine, elevated uric acid, and hypernatremia also make this unlikely. Educational objective: Polyuria in nonhospitalized patients can be due to primary polydipsia or diabetes insipidus (DI). Primary polydipsia is due to increased water intake that leads to a dilute urine (urine osmolality < 1/2 plasma osmolality) and hyponatremia (sodium <137 mEq/L). Serum Na >145 mEq/L with dilute urine excludes primary polydipsia and favors DI. DI can be central (decreased antidiuretic hormone [ADH] release from pituitary) or nephrogenic (normal ADH level but with renal ADH resistance). References: Hyponatremia and hypernatremia: disorders of water balance. (http://www.ncbi.nlm.nih.gov/pubmed/19322975) Disorders of antidiuretic hormone. (http://www.ncbi.nlm.nih.gov/pubmed/3042388)

A 53-Year-Old Man Comes to the Physician Complaining of Increased