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USMLE® Step 1 Question of the Day: Shortness of Breath

Osmosis Team
Published on May 11, 2022. Updated on May 6, 2022.

This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case focuses on a 73-year-old woman with shortness of breath. Past medical history is notable for hypertension, chronic obstructive pulmonary disease, and type II diabetes mellitus. Can you figure it out?

A 73-year-old woman presents to the emergency department with shortness of breath. She is concerned that her “COPD is flaring up.” Past medical history is notable for hypertension, chronic obstructive pulmonary disease, and type II diabetes mellitus. She has been smoking one pack of cigarettes per day for twenty years. Temperature is 37.0°C (98.6°F), pulse is 136/min, respirations are 22/min, blood pressure is 104/72 mmHg, and oxygen saturation is 92% on room air. The patient is currently speaking in three to four word sentences and demonstrates increased work of breathing. There are bilateral rales throughout the lung fields, and the electrocardiogram from triage is shown below:

Based on this patient's electrocardiogram, which of the following is the most likely diagnosis?

A. Multifocal atrial tachycardia

B. Atrial flutter

C. Paroxysmal supraventricular tachycardia

D. Atrial fibrillation with rapid ventricular response

E. Sinus tachycardia

Scroll down to find the answer!

The correct answer to today's USMLE® Step 1 Question is...

A. Multifocal atrial tachycardia

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

B. Atrial flutter

Incorrect: Atrial flutter is defined by atrial depolarization occurring at a rate of 250 to 350 beats/min. It is usually caused by an atrial reentry circuit that runs around the tricuspid annulus. The condition is characterized by "flutter waves," which have a sawtooth appearance, and the presence of multiple P-waves between QRS complexes.

C. Paroxysmal supraventricular tachycardia

Incorrect: Paroxysmal supraventricular tachycardia is defined as a regular rhythm with a rate ranging between 150 to 250 beats per minute, a narrow QRS complex, and absent P-waves. In contrast, P-waves are seen on this patient’s electrocardiogram.

D. Atrial fibrillation with rapid ventricular response

Incorrect: Atrial fibrillation with rapid ventricular response (RVR) is defined as an abnormally fast heart rate with irregularly irregular QRS complexes and absent P-waves. However, P-waves are present on this patient’s electrocardiogram, making atrial fibrillation unlikely.

E. Sinus tachycardia

Incorrect: Sinus tachycardia is characterized by a fast (>100 beats per minutes) rate, regular rhythm, and an identifiable P-wave before every QRS complex. Sinus tachycardia would not, however, manifest with the variable P-wave morphologies seen on this patient’s electrocardiogram.

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Main Explanation

This patient presents with features consistent with a COPD exacerbation and an electrocardiogram that demonstrates an irregular tachycardia, with three distinct P-wave morphologies. These features are most consistent with multifocal atrial tachycardia.

Multifocal atrial tachycardia (MAT) is an electrocardiographic diagnosis that arises due to enhanced automaticity in non-sinus node sites of either the left or right atrium. On electrocardiogram, MAT manifests as an irregular rhythm with varying R-R intervals and at least 3 distinct P-wave morphologies in the same lead. MAT is commonly mistaken for atrial fibrillation. However, careful evaluation of the above ECG demonstrates varying P-wave morphologies; whereas in atrial fibrillation P-waves are absent.

Osmosis illustration of multifocal atrial tachycardia.

Nearly 60% of patients with MAT have underlying pulmonary disease, most commonly COPD. In addition, MAT is associated with heart disease, metabolic disturbances, and pulmonary embolism. Treatment is aimed at correcting the underlying cause, as the arrhythmia is often resistant to cardioversion or temporary rate control.

Major Takeaway

Multifocal atrial tachycardia (MAT) manifests as an irregular rhythm with varying R-R intervals and at least 3 distinct P-wave morphologies in the same lead. It is commonly associated with underlying pulmonary disease.


Jameson, J. L. (2018). Harrison's Principles of Internal Medicine. New York: McGraw-Hill Education. ISBN: 9781259644030.

Kastor, J.A. (1990) Multifocal atrial tachycardia. The New England Journal of Medicine. 322(24), 1713‐1717. Doi: 10.1056/NEJM199006143222405.


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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.