Electrolyte Disorders USMLE Step 1 Practice Question
A 26-year-old woman with systemic lupus erythematosus presents to the emergency department with severe headache, confusion, and blurred vision. She reports no recent medication changes, vomiting, or diarrhea. Vital signs: BP 168/110 mmHg, HR 98/min, RR 20/min, temperature 37.1°C, SpO2 98% on room air. Physical examination reveals edema in the lower extremities and periorbital region. Laboratory studies show: Serum sodium: 128 mEq/L
Serum osmolality: 248 mOsm/kg
Serum creatinine: 3.1 mg/dL
BUN: 68 mg/dL
Urinalysis: 4+ proteinuria, RBC casts, muddy brown casts
Urine sodium: 18 mEq/L
Urine osmolality: 620 mOsm/kg Which of the following mechanisms best explains this patient's hyponatremia?
Answer choices
- AHypervolemic hyponatremia from acute kidney injury with volume expansionCorrect answer
- BEuvolemic hyponatremia from SIADH secondary to lupus nephritis
- CHypovolemic hyponatremia from renal salt wasting and nephrotic syndrome
- DEuvolemic hyponatremia from primary polydipsia
- EHypovolemic hyponatremia from gastrointestinal losses
- FHypervolemic hyponatremia from hepatic cirrhosis
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