[url="http://www.abc.net.au/science/k2/homework/s95516.htm"]http://www.abc.net.au/science/k2/homework/s95516.htm[/url]
check out this short article...hopefully the link works..may give you the answer, I don't know.
I would think it could also be a lack of some vitamin. BUt that's a guess.
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