A 15-year-old male student developed frothy urine and fatigue after participating in a 10,000-meter race at school. Urinalysis showed proteinuria (1+), which normalized after one day of rest. Six months later, he again developed frothy urine with proteinuria (3+) measuring 3.2g/24h, accompanied by periorbital edema and bilateral lower limb edema. The most appropriate investigation is: