\documentclass[a4paper]{article}
\usepackage[utf8x]{inputenc}    
\usepackage[T1]{fontenc}
\usepackage{xstring}
\usepackage[francais,bloc,completemulti,correc,ordre]{automultiplechoice}    

\begin{document}

\element{use:xtring}{
\begin{question}{q:xstring}\QuestionIndicative
Just test if \texttt{xstring} can be loaded by LaTeXML.
What you think ?
\def\mystr{Hello world}
\IfSubStr{llo}{\mystr}{Yes}{No}
\end{question}
}

\end{document}
