A physician is evaluating four patients who present with pronounced polyuria and polydipsi… — USMLE Step 2 CK

A physician is evaluating four patients who present with pronounced polyuria and polydipsia. Match each patient's clinical vignette with the most likely corresponding laboratory profile observed during a diagnostic water deprivation test and subsequent administration of exogenous desmopressin.

A $48$ -year-old man who underwent transsphenoidal resection of a large craniopharyngioma $4$ weeks ago.Urine osmolality remains $<300$ mOsm/kg during water deprivation; it increases by $>50\%$ following desmopressin administration.
A $32$ -year-old woman with bipolar disorder who has been maintained on continuous lithium therapy for the past $5$ years.Urine osmolality remains $<300$ mOsm/kg during water deprivation; it increases by $<10\%$ following desmopressin administration.
A $22$ -year-old man with severe illness anxiety disorder who reports compulsively drinking $2$ liters of water every hour to 'flush out bodily toxins'.Urine osmolality rises to $>600$ mOsm/kg during water deprivation; no significant further increase occurs after desmopressin administration.
A $55$ -year-old woman with a BMI of $34$ kg/m $^2$ , frequent recurrent vaginal candidiasis, and a hemoglobin A1c of $9.8\%$ .Baseline urine osmolality is $>300$ mOsm/kg; marked glucosuria is present; polyuria persists despite adequate hydration status.

Cevap

The correct pairings accurately match the clinical etiologies of polyuria (central DI, nephrogenic DI, primary polydipsia, and osmotic diuresis) to their respective pathophysiological responses during a formal water deprivation test and desmopressin challenge.

The water deprivation test is the gold standard for distinguishing between primary polydipsia and the two forms of diabetes insipidus (DI). In primary polydipsia, fluid restriction appropriately triggers endogenous antidiuretic hormone (ADH) release, successfully concentrating the urine. If the urine fails to concentrate during fluid restriction (indicating DI), desmopressin (an ADH analog) is administered. Central DI involves impaired ADH secretion from the posterior pituitary, so the functional kidneys rapidly concentrate urine upon receiving exogenous desmopressin. Nephrogenic DI involves renal resistance to ADH, so urine remains persistently dilute despite the exogenous desmopressin. Osmotic diuresis (e.g., from severe hyperglycemia) presents with polyuria but possesses a higher baseline urine osmolality due to the heavy load of osmotically active solutes.

Adım Adım Çözüm

Analyze the patient with recent transsphenoidal surgery.

Identified as being at high risk for central diabetes insipidus.

Surgical trauma to the hypothalamus or posterior pituitary impairs antidiuretic hormone (ADH) synthesis and release, leading to an inability to concentrate urine until an exogenous ADH analog is administered.

Analyze the patient on chronic lithium therapy.

Identified as having nephrogenic diabetes insipidus.

Lithium toxicity directly interferes with the action of ADH at the renal collecting duct. Because the defect is at the receptor level, the kidneys will not concentrate urine even when given exogenous desmopressin.

Analyze the patient with compulsive excessive fluid intake.

Identified as having primary polydipsia.

Excessive fluid intake physiologically suppresses endogenous ADH. During a water deprivation test, the physiological suppression is removed, endogenous ADH is released, and the urine concentrates normally without needing exogenous desmopressin.

Analyze the patient with poorly controlled type 2 diabetes mellitus.

Identified as experiencing osmotic diuresis.

Hyperglycemia exceeding the renal threshold leads to glucosuria. The excess glucose acts as an osmotic agent, dragging water into the urine. This leads to polyuria, but the presence of glucose means the urine osmolality is characteristically higher than in true diabetes insipidus.

Anahtar Kavram

Evaluation of polyuria using the water deprivation test and desmopressin challenge

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