Severe combined immunodeficiency



Severe combined immunodeficiency



Severe combined immunodeficiency


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USMLE® Step 1 questions

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High Yield Notes

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Severe combined immunodeficiency

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USMLE® Step 1 style questions USMLE

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An 18-month-old boy is brought to the emergency department for evaluation of shortness of breath and cough. Past medical history includes two episodes of pneumonia, oral thrush, chronic diarrhea and recurrent otitis media. His vaccinations are not up to date due to frequent illnesses. Family history is non-contributory. The patient is at the 2nd percentile for height and weight. Temperature is 38.7 C (101.7 F), pulse is 151/min, respirations are 36/min and blood pressure is 70/40 mmHg. Physical examination demonstrates white patches in the oral cavity and diffuse rales of the lungs bilaterally. Laboratory results are demonstrated below.

 Complete blood count 
 Leukocyte count  8,100 /mm3 
 Platelet count  300,000/mm3 
 Lymphocytes  5% 
 CD 19+  low 
 CD 3+  absent 
 IgG  220mg/dL 
 IgA  40 mg/dL 
 IgM  15 mg/dL 

Chest X-ray demonstrates bilateral diffuse interstitial infiltrates. Which of the following is the most likely diagnosis?

External References

First Aid








Autosomal recessive disorders

severe combined immunodeficiency p. 115

Bone marrow transplant

severe combined immunodeficiency p. 115

SCID (severe combined immunodeficiency disease) p. 96, 115

adenosine deaminase deficiency as cause p. 35

lymphopenia caused by p. 431


Severe combined immunodeficiency (SCID) is a severe form of primary immunodeficiency, caused by a genetic mutation that affects the development and function of white blood cells, which are responsible for fighting off infections. It may result from an X-linked recessive defect in IL-2R (interleukin-2 receptor) gamma chain or an autosomal recessive defect in adenosine deaminase deficiency. As a result, individuals with SCID are highly susceptible to infections, particularly bacterial and viral infections, and often present with failure to thrive, chronic diarrhea, thrush, and recurrent infections. Treatment involves HSCT (hematopoietic stem cell transplantation, avoiding live vaccines, antimicrobial prophylaxis, and isolation to prevent catching an infection.


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