A child is brought to the doctor by his parents. The parents state that the boy's urine has developed an odd "tea" color. Physical examination reveals a well-behaved child who is normotensive with no evidence of discomfort or edema. Laboratory tests indicate normal renal function and normal levels of C3. The parents deny that the boy has had any recent sore throat or skin infections. They do, however, recall a recent transient gastrointestinal infection. Which of the following renal biopsy findings would best establish the diagnosis?