Last category name

Hey,
I need some help with the retrieving categories, I have allot of sub cats, and when I print out what category the post in is like cat 1, cat2 , cat 3 and so on, how can I print only the last sub category that contains the post? lets say theres 4 sub categories
-cat
–cat 1
— cat 2
—-cat 3
—— The post

How can I print only the cat 3 as with <div class="s-title"><b><?php the_category(', ') ?></b></div>