Complete the eccentric scientist's very strict (and slightly terrifying) lab safety manual by selecting the grammatically correct option.

Under no circumstances _____ the giant red button labeled "DO NOT PUSH."

The correct answer is should you press.

"Under no circumstances" is a restrictive negative phrase. When placed at the beginning of a sentence for dramatic emphasis, it forces inversion between the subject and the auxiliary/modal verb. Therefore, "you should" becomes "should you." We do not use a negative verb ("shouldn't") because "Under no circumstances" already makes the sentence negative!