A 40-year-old woman presents with fatigue, dark urine, and swelling in her lower extremities. Urinalysis reveals proteinuria (2+), hematuria with red blood cell casts, and dysmorphic red blood cells. Serum creatinine has rapidly increased from 0.9 mg/dL to 2.2 mg/dL over the past week. Which of the following is the most appropriate next diagnostic step?

The correct answer to the question "A 40-year-old woman presents with fatigue, dark urine, and swelling in her lower extremities. Urinalysis reveals proteinuria (2+), hematuria with red blood cell casts, and dysmorphic red blood cells. Serum creatinine has rapidly increased from 0.9 mg/dL to 2.2 mg/dL over the past week. Which of the following is the most appropriate next diagnostic step?" is Option C: Renal biopsy.

The other options — A) Renal ultrasound to assess kidney size, B) 24-hour urine collection for protein, D) Serum antinuclear antibody (ANA) and ANCA testing — are incorrect for this USMLE Exam Prep question. As explained above: The patient's presentation of rapidly progressive kidney injury, active urine sediment (red cell casts, dysmorphic RBCs, proteinuria), and edema strongly suggests rapidly progressive glomerulonephrit…