CNS drugs and antiepileptics: carbamazepine autoinduction: 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 · CNS drugs and antiepileptics: carbamazepine autoinduction

Sample item

A 55-year-old man with trigeminal neuralgia is stabilised on carbamazepine. Four weeks later, despite perfect adherence, his pain recurs and the serum level has fallen. What best explains this?

AInduction of its own hepatic CYP3A4-mediated metabolismBReduced absorption because of gastric hypochlorhydriaCSaturation of its metabolic enzymes at higher dosesDDisplacement from plasma albumin by an unrelated drug

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

The answer and the reasoning

A. Induction of its own hepatic CYP3A4-mediated metabolism

Carbamazepine induces CYP3A4 and other enzymes, including those that clear itself, so clearance rises over the first few weeks and the dose often has to be increased. This autoinduction also lowers levels of co-administered drugs.

Option by option

A. Induction of its own hepatic CYP3A4-mediated metabolism (correct)
Correct. Autoinduction increases its own clearance over the first weeks of therapy.
B. Reduced absorption because of gastric hypochlorhydria
No acid-dependent absorption step is involved, and absorption does not change after a month of stable use.
C. Saturation of its metabolic enzymes at higher doses
Saturable kinetics with disproportionate rises in level is the behaviour of phenytoin, and it would raise the level.
D. Displacement from plasma albumin by an unrelated drug
Displacement would raise the free fraction briefly and would not produce a steady fall in total 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.

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is A, Induction of its own hepatic CYP3A4-mediated metabolism. Carbamazepine induces CYP3A4 and other enzymes, including those that clear itself, so clearance rises over the first few weeks and the dose often has to be increased.

Why is "Reduced absorption because of gastric hypochlorhydria" not the answer?

No acid-dependent absorption step is involved, and absorption does not change after a month of stable use.

Why is "Saturation of its metabolic enzymes at higher doses" not the answer?

Saturable kinetics with disproportionate rises in level is the behaviour of phenytoin, and it would raise the level.

Why is "Displacement from plasma albumin by an unrelated drug" not the answer?

Displacement would raise the free fraction briefly and would not produce a steady fall in total level.

What concept does this question test?

It tests CNS drugs and antiepileptics: carbamazepine autoinduction in Pharmacology. This autoinduction also lowers levels of co-administered drugs.

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 CNS drugs and antiepileptics: carbamazepine autoinduction?

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.