USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 03

A 27-year-old woman presents to her primary care physician with swelling around her eyes and ankles for 1 week. She reports frothy urine but no dysuria or flank pain. She has no significant past medical history and takes no medications. Temperature is 36.8°C (98.2°F), blood pressure is 128/78 mm Hg, pulse is 76/min, and respirations are 14/min. Physical examination shows periorbital edema and 1+ pitting edema at the shins. Urinalysis shows 4+ protein and no blood. Serum albumin is 2.1 g/dL (reference: 3.5–5.0 g/dL). Which of the following best explains the mechanism of this patient’s edema?