Hello everyone, could someone help me with this question, please?? The answer is d, but I dont know why. I am Little bit confused. Thanks in advance
(NBME form 16) A 35 year-old woman is given 500 mg of drug X intravenously. Several minutes later, the serum concentration of drug X is 12,5 mg/L. Two hours later, the serum concentration is 10 mg/L. assuming that drug X has first-order elimination kinetics, which of the following will be the serum concentration of drug X (in mg/L) in another 2 hours?
a) 6
b) 7
c) 7,5
d) 8
e) 8,5

In first order kinetics, constant fraction of a drug is eliminated per half life of that drug . Initial plasma concentration of the mentioned drug is 12.5 mg/L and after 2 hours (drug's half life) it is decreased to 10 mg/L.
You can see that 20% of drug is cleared from the plasma. For the next half life, 20% of 10 mg/lit will be cleared. 20% of 10 mg/L is equal to 8 mg/L.

thank you friend!!