Pharmacokinetics: protein binding: 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: protein binding

Sample item

A 30-year-old woman with nephrotic syndrome on phenytoin has nystagmus and ataxia, but her total serum phenytoin is 8 mg/L (usual range 10 to 20). What is the correct interpretation?

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

The answer and the reasoning

D. The free fraction is raised, so the total level underestimates active drug; measure free phenytoin

Phenytoin is highly bound to albumin. In hypoalbuminaemia the free (active) fraction rises, so a given total level represents more active drug and toxicity can occur at a low total concentration. A free level, or an albumin-corrected value, should guide dosing.

Option by option

  • A. The symptoms are unrelated to phenytoin since the level is subtherapeutic

    Nystagmus and ataxia are classic phenytoin toxicity signs and can occur at a low total level here.

  • B. She is under-dosed, so increase the phenytoin dose

    Increasing the dose would worsen the toxicity already present.

  • C. Hepatic induction has lowered the level and caused withdrawal signs

    Induction does not explain nystagmus and ataxia, which are signs of excess, not withdrawal.

  • D. The free fraction is raised, so the total level underestimates active drug; measure free phenytoin (correct)

    Correct. Low albumin raises the free fraction, and the total level is misleading.

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, The free fraction is raised, so the total level underestimates active drug; measure free phenytoin. Phenytoin is highly bound to albumin.

Why is "The symptoms are unrelated to phenytoin since the level is subtherape…" not the answer?

Nystagmus and ataxia are classic phenytoin toxicity signs and can occur at a low total level here.

Why is "She is under-dosed, so increase the phenytoin dose" not the answer?

Increasing the dose would worsen the toxicity already present.

Why is "Hepatic induction has lowered the level and caused withdrawal signs" not the answer?

Induction does not explain nystagmus and ataxia, which are signs of excess, not withdrawal.

What concept does this question test?

It tests Pharmacokinetics: protein binding in Pharmacology. In hypoalbuminaemia the free (active) fraction rises, so a given total level represents more active drug and toxicity can occur at a low total concentration. A free level, or an albumin-corrected value, should guide dosing.

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: protein binding?

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.