USMLE® Step 1 style questions USMLE
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A 6-month-old male infant comes to the physician because of recurrent ear and respiratory infections. His mother says that he has been having difficulty breastfeeding since birth and is underweight. She is worried that his illness is because she did not receive prenatal care until she was six months pregnant. She denies cigarette, drug, and alcohol use during pregnancy. Physical examination of the patient shows an underweight infant with mild central cyanosis. Examination of the head shows low-set ears, wide-set eyes, and throat examination shows a bifid uvula (see image below). Auscultation shows a normal S1 and a fixed split S2. Musculoskeletal examination shows generalized poor muscle tone. Which of the following tests is most appropriate to confirm the diagnosis based on the patient's symptoms?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
The name DiGeorge syndrome isn’t the most descriptive name, which is why it’s often also referred to as 22q11.2 deletion syndrome, which is actually pretty descriptive, and describes a condition in which a small portion of chromosome 22 is deleted, which causes a bunch of developmental abnormalities and complications.
Alright so our chromosomes are composed of genes, right?
Which are essentially instructions for everything from development to day-to-day survival, and these genes are spread out across 23 pairs of chromosomes.
22q11.2 is like an address, so 22 stands for chromosome 22, with q designating the long arm of the chromosome, then it’s on region 1, band 1, and sub-band 2.
This portion of dna, 22q11.2, spans about 30 genes and 1.5 to 3 million base pairs, which classifies it as a microdeletion since it’s less than 5 million base pairs.
Even though this region is relatively small, it encodes for some really important genes, one of which is the TBX1 gene, which is thought to play a big role in the disease.
The TBX1 gene is involved in normal development of the pharyngeal pouches, specifically pouch 3 and 4, which are fetal structures that develop into parts of the head and neck.
The third pharyngeal pouch goes on to develop into the thymus and the inferior parathyroid gland, the fourth pouch goes on to develop into the superior parathyroid gland.
So with a 22q11.2 deletion and therefore no TBX1 gene, the thymus and parathyroid gland both end up underdeveloped, called hypoplasia.
If someone has thymic hypoplasia and thymic dysfunction, the T cells don’t mature, and so these people often have a deficiency in mature T cells.
It turns out, though, that most people with DiGeorge syndrome have mild to moderate thymic dysfunction, called partial DiGeorge syndrome, which means that the immunodeficiency isn’t life-threatening.