On the 0-dimensional irreducible components of the singular locus of   $\Cal A_g$
  In this short note we give a characterization of extremal principally polarized abelian varieties determining an isolated point in $Sing \Cal A_g$. The case $g=5$ is treated with detail. 
V. Gonzalez -Aguilera, J.M. Munoz-Porras and Alexis G. Zamora