As John Burgess says, it's 100% pure beef. They have no need to add any fillers. The amount of beef in quarter pounder costs them less than $.40; in a regular hambuger it costs less than twenty cents. That's for a burger they're going to sell for about $2 and $1, respectively. It's just not worthwhile for them to shave costs there.
The meat is perfectly ordinary grades of meat for ground beef. It's USDA inspected. It's not graded, but the grading system is largely about fat content, and they're controlling the fat content in the grinding. It's not prime rib; you don't turn prime rib into burgers. Burgers are made by combining relatively tough, flavorless cuts with high-fat flavorful cuts and grinding it to give a pleasant texture. This is the same way you'd get a burger anywhere else.
McDonald's used to use the "pink slime" (meat scraped from the bones, treated with antimicrobials such as ammonia). But the stuff literally has a bad name, and they gave it up a couple of years ago. The stuff wasn't nearly as terrifying as it was made out in the media, especially not compared to the other abuses that go into every steak you've ever bought at the grocery store (which remains a chunk of muscle sliced from a dead animal), and it was still 100% dead cow flesh, but the bad publicity just wasn't worth the cost savings.
There's nothing weird about McD's beef (at least, nothing weirder than you get at the grocery store). It's just cow parts. They sell it cheap because they purchase it in bulk.
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