\documentclass[border=10pt]{standalone}
\usepackage[siunitx, RPvoltages]{circuitikz}
\begin{document}
\begin{circuitikz}[line width=3pt,european]
\draw (0,0) to[R]++(2,0)to[R]++(0,2)--++(-2,0)to[R,-.]++(0,-2);
\draw[red,line width=1pt] circle(2mm);
\end{circuitikz}
\end{document}