American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the defining characteristic of selective IgA deficiency?

1. A serum IgA level greater than 7 mg/dL

2. A serum IgA level less than 7 mg/dL

3. A serum IgA level of exactly 0.07 g/L

4. Immunoglobulin G deficiency

The correct answer is: A serum IgA level less than 7 mg/dL

Selective IgA deficiency is characterized by an abnormally low level of immunoglobulin A (IgA) in the serum. Specifically, a serum IgA level less than 7 mg/dL is indicative of this condition. Individuals with selective IgA deficiency may remain asymptomatic but can have an increased risk of infections, particularly at mucosal surfaces, due to the crucial roles that IgA plays in mucosal immunity. The condition is defined strictly by the measurement of serum IgA levels, so other options suggesting levels greater than or equal to 7 mg/dL do not accurately reflect the diagnostic criteria. Additionally, while immunoglobulin G (IgG) deficiency may occur in patients with various immunodeficiencies, it is not a defining feature of selective IgA deficiency itself, which specifically highlights the lack of IgA only. Therefore, the correct answer emphasizes the hallmark low level of IgA that defines this immunodeficiency.