The nurse administers 400 mg of an oral drug at 4 pm. The drug has a half-life of 2 hours. At what time will 100 mg remain in the client's system?

A. 1800: The medication has a half-life of 2 hours, meaning half of the drug amount is eliminated every 2 hours. At 4:00 pm the client receives 400 mg, and after the first half-life at 6:00 pm the amount remaining would be 200 mg. Since the question asks when only 100 mg remains, 6:00 pm occurs too early in the elimination process. B. 2000: After the first half-life (2 hours), the drug decreases from 400 mg to 200 mg at 6:00 pm. After the second half-life (another 2 hours), the amount is reduced by half again from 200 mg to 100 mg. This occurs at 8:00 pm, which is four hours after administration, making 2000 the correct time when 100 mg remains in the body. C. 1600: 4:00 pm is the exact time the medication is administered, meaning the full 400 mg dose is still present in the system. Drug elimination has not yet occurred because the first half-life period has not passed. Therefore, the amount of medication in the body at this time remains unchanged. D. 1400: 2:00 pm occurs before the medication was given, making it impossible for any portion of the drug to already be present in the client’s system at that time. Drug concentration calculations must always begin from the time of administration, which in this scenario is 4:00 pm.