HY USMLE Q#20 Resp/Biochem Step 1

HY USMLE Q#20 ( Embryology Step 1 KAPLAN instagram) 

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A 80-year-old man comes with shortness of breath, dizziness and fainting spells. Physical exam reveals distended neck veins and peripheral edema. A “plopping” sound is heard on auscultation of the heart. An echocardiogram shows a large mass from the smooth part of the right atrium. The region of the atrium is derived from which embryonic structure ?

A. Bulbus cordis
B. Primitive atrium 
C. Primitive ventricle 
D. Sinus Venosus 
E. Truncus arteriosus

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✅ Correct Answer D = Sinus Venosus 

Education Objective: 
Sinus means cavity, and venous refers to the venous system; hence, it will lead to the venous supply coming here. It would give rise to the smooth part of the atrium. 
❌ Choice A: Bulbus Cordis 
Bulbus = bulb and Cordis = heart 💜 
Cordis = the heart 
The bulbous is a round swelling just before the truncus arteriosus. This eventually contributes to the conus arteriosus and aortic vestibule, which are the smooth parts of the ventricles. 
❌ Choice B: The primitive atrium gives rise to the trabeculated (rough) part of the atrium. 
❌ Choice C: The Primitive Ventricle gives rise to the trabeculated (rough) part of the ventricles. 
❌ Choice E: Truncus arteriosus gives rise to the ascending aorta or pulmonary trunk.