  It is shown that, for any given $p\ge5$, $A>0$ and $B>0$, the exact upper
bound on $\mathsf{E}|\sum X_i|^p$ over all independent zero-mean random
variables (r.v.'s) $X_1,\ldots,X_n$ such that $\sum\mathsf{E}X_i^2=B$ and
$\sum\mathsf{E}|X_i|^p=A$ equals $c^p\mathsf{E}|\Pi_{\lambda}-\lambda|^p$,
where $(\lambda ,c)\in(0,\infty)^2$ is the unique solution to the system of
equations $c^p\lambda=A$ and $c^2\lambda=B$, and $\Pi_{\lambda}$ is a Poisson
r.v. with mean $\lambda$. In fact, a more general result is obtained, as well
as other related ones. As a tool used in the proof, a calculus of variations of
moments of infinitely divisible distributions with respect to variations of the
L\'{e}vy characteristics is developed.
