Complex sentence: Some dermatologists posit that if a sufferer is allergic to a substance, then a general allergy test on the forearm will suffice, yet others believe that in conditions like dyshidrosis, the suspect substances need to be applied to the affected area to induce a reaction.
Split into simpler sentences: Some dermatologists posit that, if a sufferer is allergic to a substance, then a general allergy test on the forearm will suffice. However, others believe that, with conditions like dyshidrosis, the suspect substances must be applied to the affected area to induce a reaction.