  The one-instanton contribution to the prepotential for N=2 supersymmetric
gauge theories with classical groups exhibits a universality of form. We
extrapolate the observed regularity to SU(N) gauge theory with two
antisymmetric hypermultiplets and N_f \leq 3 hypermultiplets in the defining
representation. Using methods developed for the instanton expansion of
non-hyperelliptic curves, we construct an effective quartic Seiberg-Witten
curve that generates this one-instanton prepotential. We then interpret this
curve in terms of an M-theoretic picture involving NS 5-branes, D4-branes,
D6-branes, and orientifold sixplanes, and show that for consistency, an
infinite chain of 5-branes and orientifold sixplanes is required, corresponding
to a curve of infinite order.
