How would you evaluate and treat an older teenage patient with severe perioral dermatitis with histology demonstrating a granulomatous dermatitis and all stains have come back negative? 

Minocycline (rarely use but for this, I would) with weekly ivermectin and Elidel. I have done a prednisone taper as well to calm it down.

Isotretinoin or dapsone...

Over the years, I have struggled with a handful of similar patients. Tetracyclines and dapsone are of minimal help. Isotretinoin, however, can be very effective at 0.25 mg/kg till clear. The pathophysiology remains a mystery.