  We map out possible extensions of the MSSM in the context of type II string
theory. We systematically investigate three-stack and four-stack quivers which
realize the MSSM spectrum with the addition of a single MSSM singlet S with an
allowed S H_u H_d term, which can lead to a dynamical electroweak-scale
mu-term. We present the three quivers which satisfy stringent string-theoretic
and phenomenological constraints, including the presence of non-zero masses for
all three families of quarks and leptons, the perturbative and non-perturbative
absence of R-parity violating couplings and rapid dimension-five proton decay,
and a mechanism for small neutrino masses. We find that these quivers can
realize many models in the class of singlet-extended (supersymmetric) standard
models, as D-instanton effects can in principle generate a superpotential of
the form f(S), where f is a polynomial. Finally, we address the issue of the
stabilization and decoupling of charged moduli which generically appear in
D-instanton corrections to the superpotential.
