General pharmacology and pharmacokinetics: crossover trial design: 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 and pharmacokinetics: crossover trial design

Sample item

In a randomised trial comparing two antihypertensive drugs, each patient takes drug A for 4 weeks, then drug B for 4 weeks, with a 2-week drug-free gap between the periods. The order is randomised. Why is the gap included?

ATo keep the observers blind to the allocationBTo let the first drug's effect wear off so it does not carry overCTo raise the sample size without recruiting more patientsDTo allow a placebo run-in to select responders

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

The answer and the reasoning

B. To let the first drug's effect wear off so it does not carry over

In a crossover design each patient serves as their own control, which needs a washout period long enough for the first treatment's effects to disappear. Without it, a carry-over effect biases the comparison of the second period. Crossover designs suit stable chronic conditions and cannot be used for curative treatments.

Option by option

A. To keep the observers blind to the allocation
Blinding depends on identical-looking treatments, not on a drug-free interval.
B. To let the first drug's effect wear off so it does not carry over (correct)
Correct. The washout prevents carry-over of the first drug into the second period.
C. To raise the sample size without recruiting more patients
Crossover designs need fewer patients because of paired comparison, but the gap does not itself raise numbers.
D. To allow a placebo run-in to select responders
A placebo run-in comes before randomisation and is not the purpose of a gap between treatments.

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 B, To let the first drug's effect wear off so it does not carry over. In a crossover design each patient serves as their own control, which needs a washout period long enough for the first treatment's effects to disappear.

Why is "To keep the observers blind to the allocation" not the answer?

Blinding depends on identical-looking treatments, not on a drug-free interval.

Why is "To raise the sample size without recruiting more patients" not the answer?

Crossover designs need fewer patients because of paired comparison, but the gap does not itself raise numbers.

Why is "To allow a placebo run-in to select responders" not the answer?

A placebo run-in comes before randomisation and is not the purpose of a gap between treatments.

What concept does this question test?

It tests General pharmacology and pharmacokinetics: crossover trial design in Pharmacology. Without it, a carry-over effect biases the comparison of the second period. Crossover designs suit stable chronic conditions and cannot be used for curative treatments.

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 and pharmacokinetics: crossover trial design?

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.