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USMLE Step 1

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Chromosomal Abnormalities

Down Syndrome (Trisomy 21)

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Down Syndrome, also known as Trisomy 21, is a genetic disorder caused by possessing three copies of chromosome 21. This is most often caused by meiotic nondisjunction (95%), although Robertsonian translocation (4%) or mosaicism (1%) are less common causes.

Down Syndrome is characterized by a number of clinical findings, such as prominent epicanthal folds (with flat facies), simian crease of the palm, and Brushfield spots. Patients with Down Syndrome are at increased risk for developing leukemias (AML and ALL), lymphoma, Alzheimer's disease (dementia), Hirshsprung disease, duodenal atresia, and congenital heart abnormalities.

Prenatal screening shows decreased AFP, estriol, and PAPP-A, but increased inhibin A and beta-hCG. Fetal ultrasound shows nuchal translucency and increased fluid behind the neck.

Find Down Syndrome and more Chromosomal Abnormalities among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.