General pharmacology: therapeutic index: 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: therapeutic index

Sample item

A drug has an LD50 of 400 mg/kg in animals and an ED50 of 20 mg/kg. What is its therapeutic index, and what does the value indicate?

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

The answer and the reasoning

D. 20, a ratio of the median lethal to median effective dose

Therapeutic index is the ratio LD50 divided by ED50, here 400 divided by 20, giving 20. A larger index suggests a wider safety margin, though a stricter measure, the certain safety factor, uses LD1 over ED99.

Option by option

  • A. 8000, the product of the two doses

    A product has no pharmacological meaning here.

  • B. 380, the difference between the doses

    The index is a ratio, not a difference between doses.

  • C. 0.05, since the effective dose is smaller

    This inverts the ratio; the index is lethal dose over effective dose.

  • D. 20, a ratio of the median lethal to median effective dose (correct)

    Correct. 400 divided by 20 equals 20.

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 D, 20, a ratio of the median lethal to median effective dose. Therapeutic index is the ratio LD50 divided by ED50, here 400 divided by 20, giving 20.

Why is "8000, the product of the two doses" not the answer?

A product has no pharmacological meaning here.

Why is "380, the difference between the doses" not the answer?

The index is a ratio, not a difference between doses.

Why is "0.05, since the effective dose is smaller" not the answer?

This inverts the ratio; the index is lethal dose over effective dose.

What concept does this question test?

It tests General pharmacology: therapeutic index in Pharmacology. A larger index suggests a wider safety margin, though a stricter measure, the certain safety factor, uses LD1 over ED99.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: therapeutic index?

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.