Select box with dynamic textfield

Ok, I've got a table with names, id's, and descriptions. What I would like to do is that when I select a name in a select box the description-text will shown in an textfield. This is what I've got now (It only outputs the dropdown menu):
   
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$question_one = array();
$question_one[] = array('id' => '2', 'text' =>ANSWER_DEFAULT);
$question_one_querry = tep_db_query("select sources_id, sources_name from " . TABLE_QUESTIONS . " where question_number =1 order by sources_name");
while($question_one_values =  tep_db_fetch_array($question_one_querry)) {
                           $question_one[] = array('id' =>  $question_one_values['sources_name'],
                           'text' => $question_one_values['sources_name']);
 }

 echo tep_draw_pull_down_menu('', $question_one);

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The tep_draw_pull_down_menu function looks like this:
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 function tep_draw_pull_down_menu($name, $values, $default = '', $parameters = '', $required = false) {
    $field = '<select name="' . tep_output_string($name) . '"';

    if (tep_not_null($parameters)) $field .= ' ' . $parameters;

    $field .= '>';

    if (empty($default) && isset($GLOBALS[$name])) $default = stripslashes($GLOBALS[$name]);

    for ($i=0, $n=sizeof($values); $i<$n; $i++) {
      $field .= '<option value="' . tep_output_string($values[$i]['id']) . '"';
      if ($default == $values[$i]['id']) {
        $field .= ' SELECTED';
      }

      $field .= '>' . tep_output_string($values[$i]['text'], array('"' => '&quot;', '\'' => '&#039;', '<' => '&lt;', '>' => '&gt;')) . '</option>';
    }
    $field .= '</select>';

    if ($required == true) $field .= TEXT_FIELD_REQUIRED;

    return $field;
  }
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