An 11-year-old girl was hospitalized with complaints of weakness and facial edema, observed for 3 days. The mother claims that her daughter had always been healthy and active until these symptoms started. Physical examination reveals generalized facial edema and pitting edema on the legs (pressing the edema leaves an imprint that gradually smoothes out). During the patient's history taking, the girl describes a foamy appearance of her urine but denies blood in urine, nocturia, or pain during urination. Laboratory testing detects proteinuria and microhematuria. What is the most likely cause of the changes detected in the urinalysis?