USMLE® Step 1 style questions USMLE
A 3-year-old boy is brought to the office for evaluation. He has had recurrent episodes of otitis media and respiratory infections that have required antibiotic treatment. His fine motor skills are within normal limits. He appears unsteady when asked to stand still, but can walk quickly while he plays. He displays nystagmus during finger tracking. Which of the following findings is most likely present in this patient?
So, thymic hypoplasia is a condition where the thymus is underdeveloped and has a reduced number of cells.
By week 4 of development, the embryo takes on a more recognizably “human” form—but to be honest, it still looks more like a shrimp than a baby.
The components of the pharyngeal apparatus develop into various head and neck structures, and sometimes multiple arches join together to give rise to a single structure.
Now, the epithelial tissue of the embryo’s third and fourth pouch turns into the inferior parathyroid glands and superior parathyroid glands, while the epithelial tissue that lines the ventral region of the third pouch forms the thymus.
Both glands then go on to break off from the pharyngeal wall and eventually attach to the posterior side of the thyroid.
The thymus now free, migrates down the middle of the pharynx, until it ends up in its final position in the front of the thorax where it fuses with its counterpart from the opposite side.
During childhood, the thymus occupies considerable room behind the sternum, in a part of the chest known as the mediastinum, a space in the chest between the lungs that also contains the heart.
On chromosome 22, there’s a gene called TBX1 that controls the development of the 3rd and 4th pharyngeal pouch during the prenatal period.
So, when there’s a 22q11.2 deletion, there’s no TBX1 gene, and the thymus and parathyroid glands ends up underdeveloped.
In thymic hypoplasia, the immature T cells from the bone marrow don’t have a place to go to mature, and so these people often have a deficiency in mature T cells.
The parathyroid glands also remains underdeveloped.
But when the parathyroid glands are underdeveloped, there’s less parathyroid hormone which results in hypocalcemia.
Thymic hypoplasia also happens in people with ataxia-telangiectasia syndrome, and there It's caused by a defect in the ATM gene, or Ataxia-Telangiectasia Mutated gene, located on chromosome 11.
The ATM gene encodes a protein whose job is to recognize DNA damage and activate proteins to fix the damage.