We prove that many pretzel knots of the form K = P (2n, m, −2n± 1, −m) are not topologically slice, even though their positive mutants P (2n, −2n ± 1, m, −m) are ribbon. We use the sliceness obstruction of Kirk and Livingston [KL99a] related to the twisted Alexander polynomials associated to prime power cyclic covers of knots.1 That is, such that P (p, q, r) is not a 2-bridge knot.