This is related MDL-5324 issue: allow negative score for individual questions (like SAT MCAT etc.)

If negative points apply for wrong choices in multichoice questions, then the student should have the option to leave the question blank, unanswered and avoid penalty. However, now once the student has clicked on a radiobutton, there is no way to uncheck it. The only possibility is to mark another option, but it is imposiible to leave the question blank.

A liitle javascript could help to this task. At ULPG we are using:

<?php if (!$answer->feedback) { ?>
<br /><br />
<input type=button value="<?php print_string('noresponse', 'quiz') ?>" onClick='blankresponse(<?php echo $question->name_prefix ?>)'>
<script>
function blankresponse(resp)
{
for (var i=0;i<resp.length;i++)

}
</script>

We have included these lines in question/type/multichoice/display.html form,
by line 11, aftter
<div class="ablock clearfix">
<div class="prompt">
<?php echo $answerprompt; ?>

In this way students can give a second thought and uncheck if desired.