Cardiovascular system: Einthoven law: practice question

A Physiology 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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Physiology · Cardiovascular system: Einthoven law

Sample item

On a normal ECG, the net QRS deflection is +4 mm in lead I and +6 mm in lead III. What is the net QRS deflection in lead II?

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

The answer and the reasoning

B. +10 mm

Einthoven law states that lead II equals lead I plus lead III. With +4 and +6 mm, lead II is +10 mm. The relation holds because the three limb leads form a closed triangle of potentials.

Option by option

  • A. +5 mm

    This averages the two leads, which the law does not use.

  • B. +10 mm (correct)

    Correct. Lead II = lead I + lead III = 4 + 6 = 10 mm.

  • C. +2 mm

    This subtracts the leads, which would give lead III minus lead I.

  • D. -2 mm

    This is the negative of the difference and does not follow Einthoven law.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, +10 mm. Einthoven law states that lead II equals lead I plus lead III.

Why is "+5 mm" not the answer?

This averages the two leads, which the law does not use.

Why is "+2 mm" not the answer?

This subtracts the leads, which would give lead III minus lead I.

Why is "-2 mm" not the answer?

This is the negative of the difference and does not follow Einthoven law.

What concept does this question test?

It tests Cardiovascular system: Einthoven law in Physiology. With +4 and +6 mm, lead II is +10 mm. The relation holds because the three limb leads form a closed triangle of potentials.

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 Cardiovascular system: Einthoven law?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.