I was wondering the same thing. I never did hear an explanation from the defense.
Never mind. I saw the answer several posts ago.
Maybe the ME ran a sloppy shop. He did not do in-depth autopsy on GZ.
His underlings did the grunt work. If he didn't see anything wrong, he simply did not test for it. And, if evidence was not collected, it was not his job.
That ME is a major problem. His testimony was bizarre as he described the autopsy protocol (which he made sure to say, he didn't write) - to say sloppy is not enough.