Sounds like you need a break function. Try this.

function count_down()
{
while(seconds > 0)
{
if(correct_answer_is_selected) { break; }
sleep (1);
seconds -= 1;
temp = int(seconds / 60); // compute the number of minutes
str_for_num(time_str, temp);
str_cat (time_str, " : ");
temp_seconds = seconds - temp * 60; // compute the remainder (the number of sec)
str_for_num(temp_str, temp_seconds);
if (temp_seconds < 10)
{
str_cat (time_str, "0"); // add a zero (if needed)
}
str_cat (time_str, temp_str);
}
stop_count_down();
}