Pharmacokinetics: 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

Sample item

A patient on phenytoin 300 mg/day has a plasma level of 10 mg/L. The dose is raised to 400 mg/day and the level rises to 25 mg/L with nystagmus and ataxia. What explains the disproportionate rise?

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

The answer and the reasoning

A. Saturable (zero-order) metabolism at therapeutic concentrations

Phenytoin metabolism by hepatic enzymes saturates within the therapeutic range, shifting from first-order to Michaelis-Menten kinetics. Near saturation a small dose increase gives a large rise in steady-state concentration, so dose increments must be small and levels monitored.

Option by option

  • A. Saturable (zero-order) metabolism at therapeutic concentrations (correct)

    Correct. Saturated enzymes cannot clear the extra drug, so concentration rises steeply.

  • B. Rapid first-order renal clearance

    First-order clearance would give a proportional, not disproportionate, rise.

  • C. Reduced oral bioavailability

    Reduced bioavailability would lower, not raise, the plasma level.

  • D. Increased hepatic blood flow

    Increased hepatic blood flow would increase clearance and lower the level.

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.

More Pharmacology questions

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is A, Saturable (zero-order) metabolism at therapeutic concentrations. Phenytoin metabolism by hepatic enzymes saturates within the therapeutic range, shifting from first-order to Michaelis-Menten kinetics.

Why is "Rapid first-order renal clearance" not the answer?

First-order clearance would give a proportional, not disproportionate, rise.

Why is "Reduced oral bioavailability" not the answer?

Reduced bioavailability would lower, not raise, the plasma level.

Why is "Increased hepatic blood flow" not the answer?

Increased hepatic blood flow would increase clearance and lower the level.

What concept does this question test?

It tests Pharmacokinetics in Pharmacology. Near saturation a small dose increase gives a large rise in steady-state concentration, so dose increments must be small and levels monitored.

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?

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.