Step 1: Understanding the Question:
We need to find which of the four drugs listed does not require routine therapeutic drug monitoring.
Step 2: Key Concept:
Therapeutic drug monitoring is reserved for drugs with a narrow therapeutic index, meaning the gap between an effective dose and a toxic dose is small, or for drugs whose blood levels are hard to predict from the dose alone.
Step 3: Detailed Explanation:
Digitoxin carries a real risk of dangerous heart rhythm effects if its blood level climbs too high, so it needs regular monitoring.
Phenytoin's blood levels rise unpredictably once a certain dose is passed, so tracking its levels helps avoid toxicity.
Gentamicin can damage the kidneys and hearing at high blood levels, making monitoring routine, especially in patients with reduced kidney function.
Acetaminophen, by contrast, has a wide safety margin between its effective and toxic doses when taken as recommended, and its levels behave predictably, so it does not need the same ongoing monitoring, aside from the separate situation of a suspected overdose.
Step 4: Final Answer:
Acetaminophen is the drug that does not require therapeutic drug monitoring.