A child is taken to a pediatrician because his mother notices that his eyes appear very puffy. The mother said that the boy's eyes appeared normal two days ago, and pa of what caused her concern was that her child seemed to be rapidly becoming ill. On physical examination, the boy is noted to have generalized edema. No hypeension or jaundice is noted. Blood urea nitrogen and serum creatinine are within normal limits. A urine sample is collected, and the nurse notices that the top of the urine has a small amount of foam at the top. Urinalysis is negative for glucose, red cells, white cells, casts, crystals, and bacteria. A 24-hr-urine specimen is collected, which demonstrates proteinuria of 55 mg/h/m2. Which of the following is the most likely diagnosis?