I need to add some custom markup inside the first list-item of the main-menu as you can see here:

<div id="block-system-main-menu">     <div class="block-inner clearfix">         <div class="content clearfix">             <ul class="menu">                 <li class="first leaf"><a href="/de/t1">Item 1</a>                     <div class="my-custom-markup>                         …                         …                     </div>                 </li>                 <li class="leaf"><a href="/de/t2">Item 2</a></li>                 <li class="last leaf"><a href="/de/t3">Item 2</a></li>             </ul>         </div>     </div> </div> 

With the following code, I can insert the markup:

function mytheme_preprocess_menu_link(&$  variables) {   $  variables['element']['#below']['#markup'] = ' <div class="my-custom-markup">...</div>'; } 

But this code adds the markup to every list-item, not just the first one. Do I use the appropriate function and how can I add the markup only at the first item?