  The notion of a congruence pair for principal MS-algebras, simpler than the
one given by Beazer for $K_2$-algebras \cite{6}, is introduced. It is proved
that the congruences of the principal MS-algebras $L$ correspond to the
MS-congruence pairs on simpler substructures $L^{\circ\circ}$ and $D(L)$ of $L$
that were associated to~$L$ in \cite{4}.
  An analogy of a well-known Gr\"atzer's problem \cite[Problem 57]{11}
formulated for distributive p-algebras, which asks for a characterization of
the congruence lattices in terms of the congruence pairs, is presented here for
the principal MS-algebras (Problem 1). Unlike a recent solution to such a
problem for the principal p-algebras in \cite{2}, it is demonstrated here on
the class of principal MS-algebras, that a possible solution to the problem,
though not very descriptive, can be simple and elegant.
  As a step to a more descriptive solution of Problem 1, a special case is then
considered when a principal MS-algebra $L$ is a perfect extension of its
greatest Stone subalgebra $L_{S}$. It is shown that this is exactly when de
Morgan subalgebra $L^{\circ\circ}$ of $L$ is a perfect extension of the Boolean
algebra $B(L)$. Two examples illustrating when this special case happens and
when it does not are presented.
