If I could only carry one first aid specific item in the wilderness, it would be an irrigation syringe.
There’s a lot that can be done with bandannas, duct tape, and paracord. A multitool, spare clothing, sleeping pad, tarp, poles – pretty much everything in a pack, including the pack itself, can be fashioned into some kind of medical implement with a little ingenuity. But cleaning a wound will always remain difficult. It also remains extremely important. Infection is both very common and very inconvenient in the wilderness, where you’re well away from definitive care.
Clean water should always available and irrigation is a simple and effective method of cleaning a wound. But water just poured over a wound won’t do much good. Pressure is needed. Occasionally you might hear people claim that you can fill up a ziploc bag with water, cut or poke a hole in one corner, and squeeze the bag to force out a stream of water. That’s certainly better than nothing, but in my experience the pressure from that is not comparable to the pressure from a syringe. With an irrigation syringe, you can take the cleanest water available (usually your drinking water) and shoot it into the wound. Pressure washing the wound like this allows you to easily clean out all the grit and dirt. There’s no need to go poking around in there with unsanitary tools, probably causing more harm than good. A 12cc syringe like the one I carry costs $1, weighs 8 grams (0.28 oz), and takes up very little room. I can’t think of a reason not to have one in your pack!
Remember: a clean wound is a happy wound. You can put all the effort you want into the perfect bandage, but if the wound isn’t clean, you’re going to have some problems down the line.