\documentclass{article}
\newcommand\ABc{% center aligned
\begin{array}{@{}c@{}}
A\\[-0.5ex]
\downarrow\\[-0.3ex]
B
\end{array}%
}
\newcommand\ABt{% top aligned
\begin{array}[t]{@{}c@{}}
A\\[-0.5ex]
\downarrow\\[-0.3ex]
B
\end{array}%
}
\newcommand\ABb{% bottom aligned
\begin{array}[b]{@{}c@{}}
A\\[-0.5ex]
\downarrow\\[-0.3ex]
B
\end{array}%
}
\begin{document}
\begin{itemize}
\item[$A$] Something.
\item[$\ABc$] Something, center aligned.
\item[$\ABt$] Something, top aligned.
\item[$\ABb$] Something, bottom aligned.
\item[$B$] Conclusion.
\end{itemize}
\end{document}