Final answer:
Acute acetaminophen overdose most severely affects the liver, causing hepatic failure which can lead to life-threatening conditions such as hepatic encephalopathy and impaired protein production.
Step-by-step explanation:
The most serious toxic effect of acute acetaminophen (Tylenol) overdose is hepatic failure. Acetaminophen toxicity is a leading cause of hepatic failure and can rapidly lead to liver damage and potentially to death if not treated promptly. The liver is essential for detoxifying substances and producing vital proteins, and when it is severely damaged by overdose, it cannot perform these functions. This can result in hepatic encephalopathy, a reversible neuropsychiatric abnormality due to liver failure, and disruption in the production of proteins like albumin and blood clotting factors.