not acutely become cyanotic and ill, although brief desaturations can occur
that become more persistent. The ventilator requirements are increased due to
increasing pCO 2 (as the lungs become “wet,” the pCO 2 increases). The dias-
tolic blood pressure usually drops and there is a widened pulse pressure (usu-
ally greater than 20). The PDA was always there, it is just that her pulmonary
vascular resistance relaxed enough to allow more left-to-right shunting and
more blood flow to the lungs (less to the body). An atrial septal defect (ASD),
such as a persistent foramen ovale, could be eliminated from the diagnosis
because the murmur would be heard as an abnormal splitting of the second
sound during expiration (answer a).A patent foramen ovale is a common
echo finding in premature babies and is usually not followed up unless it
appears remarkable to the pediatric cardiologist or there is a persistent mur-
mur. A patent foramen ovale might result in only minimal or intermittent
cyanosis during crying or straining to pass stool. A murmur caused by a ven-
tricular septal defect (VSD, answer c), occurs between the first and second
heart sounds (S 1 and S 2 ) and is described as holosystolic (pansystolic) because
the amplitude is high throughout systole. Pulmonary stenosis would be heard
as a harsh systolic ejection murmur (answer d).Coarctation of the aorta
(answer e)would result in a systolic murmur. PDA refers to the maintenance
of the ductus arteriosus, a normal fetal structure. In the fetus, the ductus arte-
riosus allows blood to bypass the pulmonary circulation, since the lungs are
not involved in CO 2 /O 2 exchange until after birth. The placenta subserves the
function of gas exchange during fetal development. The ductus arteriosus
shunts flow from the left pulmonary artery to the aorta. High oxygen levels
after birth and the absence of prostaglandins from the placenta cause the duc-
tus arteriosus to close in most cases within 24 hours. A PDA most often cor-
rects itself within several months of birth, but may require infusion of
indomethacin (a prostaglandin inhibitor) as a treatment, insertion of surgical
plugs during catheterization, or actual surgical ligation.
16.The answer is a.(Moore and Persaud, Developing, pp 210–211. Sadler,
pp 77–78, 263–264.)The thymic parenchyma (epithelial cells) develops from
endoderm of the third pharyngeal (branchial) pouches. The thymic rudiment
is invaded by bone marrow–derived lymphocyte precursors early in the third
month of development. The tonsils (answer b)develop as partially encapsu-
lated lymph nodules. Their parenchymal framework is derived from pharyn-
geal mesoderm. Bones, of course, whether formed by intramembranous or
endochondral ossification, are derived from mesoderm. Their forming marrow
88 Anatomy, Histology, and Cell Biology