Allergies, toxins, pathogens.These are things that you are sensitive to or make you sick. They can irritate psoriasis, but it's usually short term, and it's not from metabolic syndrome.
Most people are well aware of these, and don't need to be told to avoid them. Allergy and blood tests can be very useful for detecting the ones you may not know you have. But in general most of these are minor psoriasis concerns, besides major chronic diseases or accumulative poisons.