\usepackage{tikz}
% although the tikz code below uses the rectangle command, the outcome 
% looks very much like a circle in stead of the oblong oval you
% expect (if height > width).
% Code form automultiplechoice.dtx svn version 1287.
% I commented out the rendering code in the AMC@shape@oval code
% and replaced it with a usebox.
% The blob of ink (actually at the moment the outline only) is
% produced in the savebox command, in which all parameters are replace
% or removed. 
\AMCboxDimensions{shape=oval,width=2.2ex,height=2.8ex}
\newsavebox\amcoval
\savebox{\amcoval}{\begin{tikzpicture}%
    \useasboundingbox (-0.5\AMC@boxedwidth,0.5\AMC@boxedheight)%
    rectangle (0.5\AMC@boxedwidth,-0.5\AMC@boxedheight);%
    \draw[line width=\AMC@boxedrule,rounded corners=\AMC@oval@radius]%
    (-0.5\AMC@boxedwidth,0.5\AMC@boxedheight) %
    rectangle (0.5\AMC@boxedwidth,-0.5\AMC@boxedheight);%
    \draw (0,0) node {\phantom{A}};
  \end{tikzpicture}}
% the oval command still uses tracebox, which is needed for amc,
% but renders the 'box' using a pre-rendered  savebox.
\renewcommand\AMC@shape@oval[5]{%
\AMC@tracebox{#3}{#4}{\usebox{\amcoval}
% \begin{tikzpicture}%
% \useasboundingbox (-0.5\AMC@boxedwidth,0.5\AMC@boxedheight)
%   rectangle (0.5\AMC@boxedwidth,-0.5\AMC@boxedheight);
% \draw[#1,fill=#2,line width=\AMC@boxedrule,rounded corners=\AMC@oval@radius]
%   (-0.5\AMC@boxedwidth,0.5\AMC@boxedheight)
%   rectangle (0.5\AMC@boxedwidth,-0.5\AMC@boxedheight);
% \draw[#1] (0,0) node {#5};
% \end{tikzpicture}
}%
}
