  Let $R$ be a Dedekind ring, $\mathfrak{p}$ a nonzero prime ideal of $R$,
$P\in R[X]$ a monic irreducible polynomial, and $K$ the quotient field of $R$.
We give in this paper a lower bound for the $\mathfrak{p}$-adic valuation of
the index of $P$ over $R$ in terms of the degrees of the monic irreducible
factors of the reduction of $P$ modulo $\mathfrak{p}$. As an important
application, when the lower bound is greater than zero for some $\mathfrak{p}$,
we conclude that no root of $P$ generates a power integral basis in the field
extension of $K$ defined by $P$.
