  We show that flavor diagonal and off-diagonal susceptibilities of light
quarks at vanishing chemical potential can be calculated consistently assuming
the baryon density and isospin density dependence of QCD to be expressed by a
vector-isoscalar and a vector-isovector coupling, respectively. At the mean
field level, their expression depends only on the effective medium-dependent
couplings and quark thermodynamic potential. The strength of the couplings can
be then estimated from the model using lattice QCD data as an input.
