The three most common rashes that can start under a cast are - a) Miliaria Rubra (MR) b) Contact Allergic Dermatitis (CAD) c) Fungal/Yeast/Bacterial infection. Now all three can be itchy but the fact that your rash is dry, started on the foot and has disseminated suggests a fungal or yeast infection of the foot with a secondary allergic reaction to the infection - this often manifests as hive like symptoms. I would treat a patient like you by doing a skin scraping from the foot to look for fungal or yeast elements and if positive a 14 day course of oral Lamisil 250 mg once a day or Tablet Fluconazole 150 mg once a week for 4 weeks along with a short course of tapering oral steroids would be the best course of action. I would strongly suggest discussing this with your doctor. Till you can see him or her I would recommend to continue Benadryl and mix 1% cortizone cream with 1% clotrimazole cream in equal parts and apply twice a day on the rash. These creams are available over the counter.

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