ECG: electrical axis: 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 · ECG: electrical axis

Sample item

An ECG shows a predominantly positive QRS in lead I and predominantly negative QRS in leads II and aVF, with an axis of about minus 50 degrees and a normal QRS duration. What is the most likely cause?

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

The answer and the reasoning

D. Left anterior fascicular block

The normal frontal QRS axis lies between minus 30 and plus 90 degrees. Marked left axis deviation with a normal QRS duration, qR in I and aVL and rS in II, III and aVF, indicates block of the left anterior fascicle (hemiblock), because the left ventricle is then activated from below to above. Posterior fascicular block shifts the axis rightwards.

Option by option

  • A. Left posterior fascicular block

    Posterior fascicular block causes right axis deviation.

  • B. Normal axis variant

    An axis of minus 50 degrees lies beyond the normal limit of minus 30 degrees.

  • C. Right ventricular hypertrophy

    Right ventricular hypertrophy causes right axis deviation.

  • D. Left anterior fascicular block (correct)

    Correct. Delayed activation of the anterosuperior left ventricle swings the axis leftwards beyond minus 30 degrees.

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 D, Left anterior fascicular block. The normal frontal QRS axis lies between minus 30 and plus 90 degrees.

Why is "Left posterior fascicular block" not the answer?

Posterior fascicular block causes right axis deviation.

Why is "Normal axis variant" not the answer?

An axis of minus 50 degrees lies beyond the normal limit of minus 30 degrees.

Why is "Right ventricular hypertrophy" not the answer?

Right ventricular hypertrophy causes right axis deviation.

What concept does this question test?

It tests ECG: electrical axis in Physiology. Marked left axis deviation with a normal QRS duration, qR in I and aVL and rS in II, III and aVF, indicates block of the left anterior fascicle (hemiblock), because the left ventricle is then activated from below to above. Posterior fascicular block shifts the axis rightwards.

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 ECG: electrical axis?

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.