The collective dynamics of a simple model colloidal suspension, namely, a two-dimensional Brownian Yukawa fluid, is studied by Brownian dynamics simulations without hydrodynamic interactions. The results for the intermediate scattering function F(k,t) are employed to assess the quantitative and qualitative accuracy of the single-exponential memory function (SEXP) approximation. We find that, contrary to self-diffusion dynamics, the additional, accesorial approximation of neglecting the three-body correlations in the third moment condition, leads to unphysical predictions of the SEXP approximation. We also show that these spurious features are readily removed by including the contribution of the three-particle distribution function to the third moment of F(k,t), even in a simple approximate manner implemented in this work.