Extracting an expression matching a pattern from a large expression

I have a Mathematica expression that contains a single square root, schematically

expr = a / (b + Sqrt[c]);

where a,b,c are large expressions. I would like to extract the expression under the sqrt, for instance by matching to a pattern, something like

Match[expr,Sqrt[x_]] // should return c

Is there an easy way to do this?