General pharmacology and pharmacokinetics: half-life and elimination: practice question

A Pharmacology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Pharmacology · General pharmacology and pharmacokinetics: half-life and elimination

Sample item

A drug with first-order elimination and a half-life of 4 hours is given as a single intravenous bolus. What percentage of the dose remains in the body 16 hours later?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. 6.25 percent

Sixteen hours is four half-lives. Each half-life halves the amount, so half four times leaves one sixteenth, or 6.25 percent. After about five half-lives roughly 97 percent has gone.

Option by option

  • A. 6.25 percent (correct)

    Correct. Four half-lives leave one sixteenth of the dose.

  • B. 12.5 percent

    This is the amount left after three half-lives, 12 hours.

  • C. 25 percent

    This is the amount left after two half-lives, 8 hours.

  • D. 3.1 percent

    This is the amount left after five half-lives, 20 hours.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is A, 6.25 percent. Sixteen hours is four half-lives.

Why is "12.5 percent" not the answer?

This is the amount left after three half-lives, 12 hours.

Why is "25 percent" not the answer?

This is the amount left after two half-lives, 8 hours.

Why is "3.1 percent" not the answer?

This is the amount left after five half-lives, 20 hours.

What concept does this question test?

It tests General pharmacology and pharmacokinetics: half-life and elimination in Pharmacology. Each half-life halves the amount, so half four times leaves one sixteenth, or 6.25 percent. After about five half-lives roughly 97 percent has gone.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General pharmacology and pharmacokinetics: half-life and elimination?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.