  Quantum chromodynamics with two zero-mass flavors is expected to exhibit a
phase transition with O(4) critical behavior. Fixing the universality class is
important for phenomenology and for facilitating the extrapolation of
simulation data to physical quark mass values. Other groups have reported
results from lattice QCD simulations with dynamical staggered quarks at Nt = 4,
which suggest a departure from the expected critical behavior. We have pushed
simulations to the largest volumes and smallest quark mass to date. Strong
discrepancies in critical exponents and the scaling equation of state persist.
