When a 500-mg medical dosage has degenerated to approximately 63 mg of its original, how many half-lives have passed?

A) 4
B) 3
C) 2
D) None of the above

The half-life concept implies that the quantity of a substance halves over each half-life period. Starting with 500 mg, after one half-life, it would be 250 mg; after two, 125 mg; and after three, approximately 62.5 mg. Since the dosage has degenerated to approximately 63 mg, this indicates that about three half-lives have passed. However, the question asks for the number of half-lives passed to reach approximately 63 mg, which would be after the third half-life, making the correct interpretation four half-lives in total (including the initial state and the three reductions).