What vitamin is lacking in your body when bruises don't heal quickly?

My wife is a teacher and is always on the go. Every now and then, she'll get a bruise and it'll take weeks to completely heal. What vitamin is she lacking?
Update: ::hugs Penny::

Update 2: Thanks to everyone who answered. I'm going to buy her some Vitamins C, K, and Iron, and get her to a doc to make sure everything's on the up and up. :)
Update 3: And potassium, too.
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