Final answer:
To conclude that PQRS is a square, we need additional information confirming that all internal angles are right angles or that it is both a rectangle and a rhombus.
Step-by-step explanation:
To determine that a quadrilateral PQRS is a square, knowing that PQ=QR=RS=SP is not enough. In addition to equal sides, we need information on the angles of the quadrilateral. Specifically, we need to know that all internal angles are right angles (90 degrees). Alternatively, knowing that the quadrilateral is both a rectangle (which has equal angles) and a rhombus (which has equal sides) would suffice, since a square is essentially a rectangle with equal sides.