Patau syndrome (Trisomy 13)

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Patau syndrome (Trisomy 13)




Patau syndrome (Trisomy 13)


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Patau syndrome (Trisomy 13)

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USMLE® Step 1 style questions USMLE

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A 35-year-old primigravida comes to the office to discuss the results of abnormal laboratory tests. The patient is at 12 weeks gestation based on her last menstrual period. She has been taking prenatal vitamins consistently and has no active complaints. Past medical history is noncontributory. First trimester screening tests revealed reduced levels of β-hCG and pregnancy-associated plasma protein-A.  Ultrasound at that time demonstrated nuchal thickening. Chorionic villus sampling reveals a fetal karyotype of 47, XX, +13. The fetus is at the greatest risk of developing which of the following conditions?  

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Cleft palate p. 644

Patau syndrome p. 61

Cutis aplasia

Patau syndrome p. 61

Holoprosencephaly p. 505

Patau syndrome p. 61


Patau syndrome p. 61

Patau syndrome p. 61

cataracts p. 554

chromosome association p. 62

holoprosencephaly p. 505

Patau syndrome (trisomy NaN)

Trisomy NaN (Patau syndrome)

hCG in p. 657


Patau syndrome is a chromosomal disorder where a person inherits an extra copy of chromosome 13, or a part of it.

So instead of having two, they have three copies of chromosome 13, and so Patau syndrome is also known as trisomy 13.

Patau syndrome is named after Dr. Klaus Patau, who first described the chromosomal nature of the syndrome.

All right, our DNA is like this humongous blueprint of information on how to make a human.

Usually this massive document is packaged up nicely into storage bins called chromosomes.

Actually, we usually have 46 chromosomes that we use to neatly organize all our information, depending on how you define organize.

Each of the 46 chromosomes is actually part of a pair of chromosomes, since you get one from each parent, so 23 pairs.

If you wanted to make another human, first you’d have to find someone that feels the same way, and then you both contribute half of your chromosomes, so one from each pair, right? Fifty-fifty.

Now, what if someone contributes one too many? Say Dad contributes 23 and Mom contributes 24, is that possible?

Yes, and it’s the basis of trisomies, in other words, having three copies of a particular chromosome.

Trisomy 13, or Patau syndrome, is the least common, but most severe trisomy in live births.

Alright, so in order to package up half the chromosomes into either a sperm cell or an egg cell, you actually start with a single cell that has 46 chromosomes, let’s just say we’re making an egg cell for the mother.

I’m just going to show one pair of chromosomes, but remember that all 23 pairs do this.

So the process of meiosis starts, which is what produces our sex cells, and the chromosomes replicate, and so now they’re sort of shaped like an ‘X’—even though there are two copies of DNA here, we still say it’s one chromosome since they’re hooked together in the middle by this thing called a centromere.

OK, then the cell then splits in two, and pulls apart the paired chromosomes, so in each of these cells, you now have 23 chromosomes.

Now the two copies of the chromosome get pulled apart, and the cells split again, which means four cells, each cell still with 23 chromosomes.

Now these are ready to pair up with a sperm cell from dad that has 23 chromosomes as well, totaling 46 chromosomes, and voila–nine months down the road you’ve got yourself a baby.


Patau syndrome, also known as trisomy 13, is a chromosomal disorder where a person inherits three copies of chromosome 13, usually as a result of meiotic nondisjunction.

Patau syndrome is characterized by a wide range of physical and mental abnormalities. Symptoms may include craniofacial defects, heart defects, brain malformations, intellectual disability, and developmental delays. There is no cure for Patau syndrome, treatment focuses on managing symptoms and providing supportive care.


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