USMLE® Step 1 style questions USMLE
A mother brings her 3-week old infant to the pediatrician's office because he was not back to birth weight at the baby's initial visit or 2-week weight check, and she is concerned he is not feeding well. His temperature is 36.6°C (98°F), pulse is 120/min (normal is 100-160), respirations are 60/min (normal is 30-50), and blood pressure is 90/50 mmHg (normal is 75-100/50-70). Physical exam shows the infant is tachypnic, diaphoretic, and lethargic. He also has a cleft palate. Based on this information, which of the following is the most likely cause of this patient's presentation?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
The truncus arteriosus is a big structure that’s present during fetal development; later in development, it divides to form two separate arteries: the aorta and pulmonary artery. Once it’s fully developed, the aorta comes from the left ventricle, and the pulmonary artery comes from the right ventricle.
A persistent truncus arteriosus describes when this developmental structure doesn’t divide into the two separate arteries. Instead, the baby’s left with one giant artery that branches off from both the right and the left ventricles that then splits off into the aorta and the pulmonary artery. Sometimes, this condition is simply referred to as truncus arteriosus, or TA. The cause of TA is unknown; however, a lot of cases seem to be associated with 22q11.2 deletion syndrome, also known as DiGeorge Syndrome.
Let’s switch to a more simplified view of the heart. All right, so usually deoxygenated blood comes in from the body and travels to the right atrium; then, it goes to the right ventricle and is pumped through the pulmonary artery to the lungs to be re-oxygenated. Next, freshly oxygenated blood passes from the lungs to the left atrium, goes to the left ventricle, and gets pumped through the aorta to the body. Then that circuit repeats, right?
If these two great arteries, the aorta and the pulmonary artery, don’t divide, you essentially have this massive artery coming from both ventricles. However, notice that this one big artery does eventually split into the aorta and pulmonary artery. Even though they eventually split off, before they do, it’s just one single vessel; thus, the oxygenated and deoxygenated blood mix. When deoxygenated blood mixes into the systemic circulation, it’ll often present as cyanosis, a bluish-purple discoloration of the skin, which can be seen in a baby within the first days after birth.