Can I write the word USA on an item made in Pakistan?

Depends - does the knife say anywhere on it "made in Pakistan"? If not lets just use some common sense and ask "Why am I putting USA on the knife? Am I trying to trick any potential consumer into thinking that the knife was made in the USA?" Think of the word USA as advertising. Is what you are doing engaging in false advertising? Is it a designation of false origin? Look at the Lanham Act for your answer. I think the answer is simply "dont do it"

You may say "designed in the USA and made in Pakistan". Language that would deceive an ordinary person reading your label is arguably fraud.

You are implying that the knife is made in the USA while knowing that this is not the truth. You intend to take advantage of your customers' confusion about the source and quality of these knives which are potentially deadly products, especially if they are poorly made or manufactured in sweatshops where output is the only thing that matters. Remove the "USA" from the brand marking.