NBME 6 Question Help

[Inbox]

Q-35
A newborn is in severe respiratory distress immediately following delivery. She was born at 35 weeks' gestation to a 35-year-old woman, gravida 2, para 1, aborta 1, who did not receive prenatal care. The newborn's pulse is 60/min, and respirations are irregular and labored. Examination shows pallor with perioral cyanosis, anasarca, hepatosplenomegaly, and scattered petechiae. Cord blood hemoglobin is 4 g/dL, and reticulocyte count is 18%. A direct antiglobulin (Coombs') test is positive. Which of the following sets of blood groups is most likely in the mother and her newborn?

Mother Newborn
A) A, Rh-positive O, Rh-positive
B) A, Rh-positive O, Rh-negative
C) A, Rh-negative O, Rh-negative
D) O, Rh-positive O, Rh-negative
E) O, Rh-negative O, Rh-positive

Took the test picked B and got the answer wrong. Could someone explain why D is the answer? Thanks

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[StrangerX]

Q-35
A newborn is in severe respiratory distress immediately following delivery. She was born at 35 weeks' gestation to a 35-year-old woman, gravida 2, para 1, aborta 1, who did not receive prenatal care. The newborn's pulse is 60/min, and respirations are irregular and labored. Examination shows pallor with perioral cyanosis, anasarca, hepatosplenomegaly, and scattered petechiae. Cord blood hemoglobin is 4 g/dL, and reticulocyte count is 18%. A direct antiglobulin (Coombs') test is positive. Which of the following sets of blood groups is most likely in the mother and her newborn?

Mother Newborn
A) A, Rh-positive O, Rh-positive
B) A, Rh-positive O, Rh-negative
C) A, Rh-negative O, Rh-negative
D) O, Rh-positive O, Rh-negative
E) O, Rh-negative O, Rh-positive

Took the test picked B and got the answer wrong. Could someone explain why D is the answer? Thanks

Mom must be Rh + AND baby Rh - for baby to get problems OR Mom can be Type O AND baby non-O type (as these moms produce IgG Anti A and B which can cross the placenta vs B and A moms who produce IgM antibodies). ABO incompatibility can occur with the first pregnancy but you need to be on at least the second pregnancy for Rh incompatibility to occur. This would seem to rule out Choices A, B, C, D. It seems like Choice E is logical. Additionally the picture seen in the question seems to correspond to erythroblastosis fetalis which is more likely seen in Rh incompatibility. It seems odd that a mom with Rh+ blood would attack fetal cells which have no Rh antigen as there is nothing for the antibodies to bind to and destroy. So I'm not sure why the answer is D over E unless I am missing something obvious. Did you get the answers online?

 

[Inbox]

Thanks. Yea the answer is E. I totally confused the RH factor

 

[adrenergic91]

"She was born at 35 weeks' gestation to a 35-year-old woman, gravida 2, para 1, aborta 1, who did not receive prenatal care."
​

So, this is someone has had 1 kid and 1 abortion, and didn't get any prenatal care. We can assume the question is setting us up for something like "which of the of the following things that are NORMALLY done as part of prenatal care did she not get and is now screwing her over?"

The baby has evidence of significant hemolysis. Hemolysis in general can be due to either Rh, or ABO incompatibility. The latter is very mild, and might present only with some self-resolving jaundice, or at most requiring phototherapy. Also, there's nothing you can do about ABO incompatibility, so the set-up of "mother without prenatal care" wouldn't make sense.

With Rh incompatibility however, you have a mother who is Rh antigen [-], who has likely formed Rh antibodies during previous pregnancy (assumingly with the same guy) during which she didn't receive Rhogam. Baby is Rh antigen [+], so the mother's antibodies cross the placenta and opsonize baby's RBCs --> hemolysis.