  The quark number susceptibility, associated with the conserved quark number
density, is closely related to the baryon and charge fluctuations in the
quark-gluon plasma, which might serve as signature for the quark-gluon plasma
formation in ultrarelativistic heavy-ion collisions. Beside QCD lattice
simulations, the quark number susceptibility has been calculated recently using
a resummed perturbation theory (Hard Thermal Loop resummation). In the present
work we show, based on general arguments, that the computation of this quantity
neglecting Hard Thermal Loop vertices contradicts the Ward identity and
violates the thermodynamic sum rule following from the quark number
conservation. We further show that the Hard Thermal Loop perturbation theory is
consistent with the thermodynamic sum rule.
