  Using the ADHM instanton calculus, we evaluate the one-instanton contribution
to the low-energy effective prepotential of N=2 supersymmetric SU(N) Yang-Mills
theory with N_F flavors of hypermultiplets in the fundamental representation
and a hypermultiplet in the symmetric rank two tensor representation. For
N_F<N-2, when the theory is asymptotically free, our result is compared with
the exact solution that was obtained using M-theory and we find complete
agreement.
