In my crime thriller, one of my characters is killed while recovering from a subdural hematoma in the hospital. What kind of poison could be injected into her IV that would cause her organs to shut down and for her to die but that wouldn't render her unconscious. She should still be able to speak, before expiring? This poison shouldn't immediately show up on an autopsy but could be something a smart ME would find.
Succinylcholine is a possible candidate. It's a powerful paralytic, commonly used in surgery, that metabolizes quickly and leaves only very slight traces. Looks like heart failure to a pathologist who isn't specifically looking for death by poison.
Being a paralytic, the person would remain conscious, but incapable of moving. In high doses, it paralyzes the diaphragm, and the person suffocates. In clinical use, high doses of succinylcholine are accompanied by a respirator to breathe for the patient.
Of course, that would leave the person incapable of speaking, so it misses that part of your scenario.
However... this is authentic. Sux (as it is offen abbreviated) has been used in a couple of murders by people who have been trained in it's use. Only recently has a test been developed to find succinylcholine in a murder victim.
Paracetamol in a high enough dosage causes the liver to shut down and can kill you in around a day or so. Most people take over the counter pain killers so unless the ME actually suspected poisoning by paracetamol, they wouldn't look for it.
You can even use lesser amounts to slowly destroy the liver over time.