skin inflammation

recently my 4 months' old puppy tends to scratch excessively. her skin becomes reddened with a patch of slight inflammtion and another patch which starts to become bald. besides a possible blood test, I would also like to monitor her diet. she is presently on ANF and it lists corn but not as the number 1 ingredient. pls advice which brand (ScienceDiet,Abady,Purina ProPlan,ANF) contains least amt of corn. I have checked the Food/Bone/Allergy section b4 posting this question but was unable to determine the amt of corn used in each brand. At the moment, I am not sure if there is a corn-free product or one that is hypo-allergenic sold in my area and I do not live in the united states.