That was fun! The mechanics were competently implemented, the shooting was pretty fun, and some of the more puzzling rooms did tease my brain. The boss was also a decent challenge.
The one-item limit was interesting in concept, but mostly annoying in execution. I sort of got around it by abusing the item swap / throw mechanic to juggle two items around everywhere. To be fair, it was really fun juggling two items around places, but I'm not sure if it was intentional design.
The heavy use of antialiased text is mildly problematic, since it creates a lot of in-between colors. Not egregious, just questionable.
The audio was a tad loud, but the boss music was much louder than everything else; it's way too loud! It was quite strange.
All-in-all though, great work!