USMLE® Step 1 style questions USMLE
A 3-day-old newborn male is evaluated in the neonatal intensive care unit for breathing difficulty. The patient was born at 39 weeks gestation via vaginal delivery at home, and this is his first medical evaluation. The mother is a 26-year-old woman who had minimal prenatal care. She reports that he “was fine” after birth, but after the first few days, she noted a constant bluish color of his lips. Temperature is 37°C (98.6°F), pulse is 170/min, respirations are 65/min, and blood pressure is 80/50 mm Hg. Pulse oximetry shows an oxygen saturation of 84% on room air. Physical exam shows low-set ears and bulbous nasal tip. Cardiac auscultation reveals a loud, single second heart sound and bounding peripheral pulses. Chest radiograph shows pulmonary congestion with increased pulmonary markings and decreased soft-tissue attenuation in the right anterior mediastinum. Which of the following cardiac defects is most likely to be seen with further analysis of this patient?
Truncus arteriosus exam links
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.
Persistent truncus arteriosus (PTA) is a rare congenital heart defect in which the embryonic structure that normally divides into the pulmonary artery and aorta fails to form properly, resulting in a single large vessel that supplies blood to both the systemic and pulmonary circulations.
Symptoms of PTA may include cyanosis, rapid breathing, poor feeding, and difficulty gaining weight. If left untreated, PTA can lead to heart failure and other complications. Treatment typically involves surgical repair of the defect, which involves separating the pulmonary artery from the aorta and creating a connection between the right ventricle and the pulmonary artery.