Related post by category, but only one

Question

I'm using this code to display related post by category and it works fine if there is only one category. But in the case of multiple categories I only want to use one, the first one. How do I limit this to just one category?

 function related_posts_categories() {    
        $categories = get_the_category();
        if ($categories) {
        foreach ($categories as $category) {
        $cat = $category->cat_ID;
        $args=array(
        'cat' => $cat,
        'orderby' => 'date',
        'order'   => 'DESC',
        'posts_per_page'=>4,
        'ignore_sticky_posts'=>1
        );
        $my_query = null;
        $my_query = new WP_Query($args);
        if( $my_query->have_posts() ) {
        echo '<div id="related_posts"><h3>Related Content</h3><ul>';
        while ($my_query->have_posts()) : $my_query->the_post(); ?>
        <li>
             <a href="<?php the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>">
             <?php the_post_thumbnail( 'related-posts' ); ?>
             </a>
             <div class="related_content">
             <a href="<?php the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a>
             </div>
            </li>
         <?php
        endwhile;
        }
        echo '</ul></div>';
        }} wp_reset_query(); }