An 11-year-old girl is brought to the doctor's office by her mother who complains that her daughter presents with weakness and swollen face for 3 days. The mother states that her daughter had always been healthy and active until the onset of symptoms. Physical examination reveals generalized swelling of the face and pitting edema on the lower limbs. Upon inquiry, the girl describes a foamy appearance of her urine but denies blood in urine, nocturnal urinary frequency, or pain during urination. Laboratory study shows proteinuria and microscopic hematuria. Which of the following is the most likely cause of the findings in the laboratory study of urine?