What is in diet pepsi that would cause skin rash?

There is nothing in Diet Pepsi that has been known to cause skin rashes. Skin rashes are usually caused by allergies or infections and are not typically linked to the consumption of Diet Pepsi. If you think you may have an allergic reaction or rash that is caused by drinking Diet Pepsi, it is best to consult with a medical professional who can help identify the source of the issue.