Final answer:
A decrease in proteinuria indicates that the medication is effective.
Step-by-step explanation:
Primary nephrotic syndrome is a condition characterized by damage to the glomerulus, leading to the leakage of protein in the urine. When a school-age child with primary nephrotic syndrome is taking prednisone, a corticosteroid, one of the manifestations that indicates the medication is effective is a decrease in proteinuria. Proteinuria refers to the presence of excess protein in the urine. So, in this case, manifestation A, a decrease in proteinuria, would indicate that the medication is effective.