Ease Into Prep with a USMLE^® Question of the Day

The correct answer is D. In the fourth week of development, the heart develops initially as a ventral midline tubular structure. The tube bulges in five regions along its length: from caudal to cranial, the bulges are the sinus venosus, primitive atrium, primitive ventricle, bulbus cordis and truncus arteriosus. The smooth part of the right atrium (the sinus venarum) is derived from the sinus venosus. The coronary sinus and the oblique vein of the left atrium also derive from the sinus venosus. On the other hand, the trabeculated parts of both atria are derived from the primitive atrium.

The mass causing the "plopping" sound is most likely an atrial myxoma. Right atrial myxoma is less common than left-sided tumors but may produce symptoms related to physical obstruction of the right atrium, such as shortness of breath, dizziness, syncope, distended neck veins, and peripheral edema.

The bulbus cordis (choice A) gives rise to the smooth part of the right ventricle (conus arteriosus) and the smooth part of the left ventricle (aortic vestibule).

The primitive atrium (choice B) gives rise to the trabeculated parts of the right and left atria.

The primitive ventricle (choice C) gives rise to the trabeculated parts of the right and left ventricles.

The truncus arteriosus (choice E) gives rise to the proximal part of the ascending aorta and the pulmonary trunk. The aorticopulmonary septum divides the truncus arteriosus into the ascending aorta and pulmonary trunk.

The table below shows adult structures derived from the dilatations of the primitive heart.

*The smooth-walled part of the left atrium is formed by incorporation of parts of the pulmonary veins into its wall. The smooth-walled part of the right atrium is formed by the incorporation of the right sinus venosus.

This is a single-step non-diagnosis question.

What is the question asking?

The question asks you to identify the embryonic structure that gives rise to the smooth-walled part of the right atrium.

What relevant information is provided in the vignette?

The only relevant information in this question is that the smooth-walled part of the right atrium is identified as the chamber where the tumor is located and you are asked to identify its embryonic origin. All of the rest of the information in the vignette is unnecessary to answer the question.

What is the underlying concept that will help answer this question?

The concept being tested is that different parts of the adult heart have different embryonic origins.

• Rough-walled parts of both atria - primitive atria
• Rough-walled parts of both ventricles - primitive ventricles
• Smooth-walled part of right atrium - sinus venosus (choice D)
• Smooth-walled part of the left atrium - pulmonary vein
• Smooth-walled part of both ventricles - bulbus cordis

Can other answers be eliminated?

All choices that give rise to a structure other than the smooth-walled part of the right atrium can be eliminated.

• Bulbus cordis (choice A) gives rise to the smooth-walled parts of both ventricles.
• Primitive atrium (choice B) gives rise to the rough-walled part of both atria.
• Primitive ventricle (choice C) gives rise to the rough-walled part of both ventricles.
• Truncus arteriosus (choice E) gives rise to the ascending aorta and the pulmonary trunk.

What is the single best answer and why?

The sinus venosus (choice D) gives rise to the smooth-walled part of the right atrium, as well as the coronary sinus, and the oblique vein of the left atrium (of Marshall).

MedEssentials (4th Ed.): pp. 230, 254

First Aid (2019): pp. 278.1

First Aid (2018): pp. 274.1

First Aid (2017): pp. 268.1