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OVERVIEW
This page is dedicated to organizing various examples of standardized exam questions whose answer is severe combined immunodeficiency disease (SCID). While this may seem a odd practice, it is useful to see multiple examples of how SCID will be characterized on standardized exams (namely the boards and the shelf exams). This page is not meant to be used as a tradition question bank (as all of the answers will be the same), however seeing the classic “test” characterization for a disease is quite valuable.
KEY CHARACTERISTICS OF THIS CONDITION (ON EXAMS)
When it comes to standardized exams, each condition has its own “code” marked by key buzzwords, lab findings, clues, etc. If you are well versed in this code you will be able to more quickly identify the condition that is being discussed, and get the right answer on the exam you are taking. Below is the “code” for SCID.
- Severe bacterial and fungal infections during infancy: given the nature of this immunodeficiency, this is expected. These may begin ~ 6 months after delivery (as maternal antibodies drop in the patient).
- Chronic diarrhea is a common element of the history.
- Mucocutaenous candidiasis
- Hypogammaglobulinemia can be present
- Low/absent T cells can be seen on lab studies.
- Thymic hypoplasia or aplasia may be a finding in infants (absent thymic shadow on imaging)
QUESTION EXAMPLES
Question # 1
A 4 month old boy is brought to the physician because he has been fussy and has run a fever for the past 2 days. His past medical history is significant for a previous episode of otitis media, ora candida, and gastroenteritis due to rotavirus infection. The patient is currently below the 5th percentile for his weight. His temperature is 103.0°F, pulse is 122/min, respirator rate is 30/min, and blood pressure is 80/55 mm Hg. A physical exam shows an erythematous and bulging left tympanic membrane. The rest of the exam is not contributory. Lab studies are collected and the results are below:
Complete Blood Count:
- Platelets: 250,000/mm³
- Leukocytes: 8,000/mm³
- Lymphocytes: 6%
Immunoglobulins:
- IgG: 205 mg/dL
- IgA: 35 mg/dL
- IgM: 14 mg/dL
Lymphocytes:
- CD4+ count: 80/mm³
- CD8+ count: 75/mm³
What is the likely diagnosis in this patient?
Explanation #1
Recurrent infections + low immunoglobulins + decreased levels of T cells = SCID
Question # 2
Explanation #2
TESTABLE FACTS ABOUT THIS CONDITION (BEYOND ITS IDENTIFICATION)
Many questions on standardized exams go beyond simply recognizing the underlying condition. Often there are specific testable facts regarding some aspect of the disease’s pathophysiology/management/clinical implications that are commonly asked. Some of these are listed below:
- Cause: various mutations in different genes that result in impaired T and B cell development.
- X-linked SCID: most common
- Adenosine deaminase deficiency; second most common.
- Consequence of disease: impaired cell-mediated and humoral immunity.
- Treatment: stem cell transplant.
Page Updated: 11.26.2016