An 8-year-old boy is brought to the clinic because of a 3-day history of dark brown urine that his mother says "looks like cola." She also notes his face has appeared puffy, especially around his eyes in the mornings. Two weeks ago, he had a sore throat for which he completed a 10-day course of amoxicillin. He has been otherwise healthy and takes no other medications. His temperature is 37.0°C (98.6°F), and blood pressure is 128/82 mm Hg (>95th percentile for age). Physical examination shows mild periorbital edema. There is no suprapubic tenderness or costovertebral angle tenderness. Laboratory studies show:
Serum
Creatinine 1.1 mg/dL
Blood urea nitrogen 24 mg/dL
Urinalysis
Color: Dark brown
Blood: 3+
Protein: 2+
Leukocyte esterase: Negative
Nitrites: Negative
Microscopic examination of urine shows 30-40 RBCs/hpf with dysmorphic RBCs and RBC casts.
Which of the following is the most likely diagnosis?