4 year olds blood work shows deficiency in amino acid proline. what could be causing this?

More info?: Can you provide more information as to why this test was done. It is certainly not routine to check. Does the children have a skin or connective tissue disorder, a history of mental retardation? Is the child's diet low in meat, dairy or eggs which provide adequate source of proline? Is the family vegan - this may be the source of intake deficiency,

Is that so?: This is not a routine test. You should insist on a full explanation from whoever had you get it.