Pharmacokinetics: high extraction drugs: 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 · Pharmacokinetics: high extraction drugs

Sample item

A patient with severe congestive heart failure is given a standard lignocaine infusion for ventricular arrhythmia and develops confusion and seizures. What is the best explanation?

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

The answer and the reasoning

D. Reduced hepatic blood flow lowers the clearance of this high extraction drug

Lignocaine has a high hepatic extraction ratio, so its clearance is limited by liver blood flow rather than enzyme activity. In heart failure hepatic perfusion falls and clearance drops, so a usual infusion rate causes toxic levels. The dose must be reduced.

Option by option

  • A. Increased volume of distribution lowered the plasma concentration

    A larger distribution volume would lower, not raise, plasma concentration.

  • B. Reduced plasma albumin increased free drug only

    Albumin binding changes matter little for a flow-limited drug and do not explain the accumulation.

  • C. Induced hepatic enzymes lowered its metabolism

    Enzyme induction would increase clearance, not reduce it.

  • D. Reduced hepatic blood flow lowers the clearance of this high extraction drug (correct)

    Correct. Flow-limited clearance falls when hepatic perfusion falls, and the drug accumulates.

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, Reduced hepatic blood flow lowers the clearance of this high extraction drug. Lignocaine has a high hepatic extraction ratio, so its clearance is limited by liver blood flow rather than enzyme activity.

Why is "Increased volume of distribution lowered the plasma concentration" not the answer?

A larger distribution volume would lower, not raise, plasma concentration.

Why is "Reduced plasma albumin increased free drug only" not the answer?

Albumin binding changes matter little for a flow-limited drug and do not explain the accumulation.

Why is "Induced hepatic enzymes lowered its metabolism" not the answer?

Enzyme induction would increase clearance, not reduce it.

What concept does this question test?

It tests Pharmacokinetics: high extraction drugs in Pharmacology. In heart failure hepatic perfusion falls and clearance drops, so a usual infusion rate causes toxic levels. The dose must be reduced.

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 Pharmacokinetics: high extraction drugs?

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.