A 3-year-old boy is brought to the emergency department for 2 days of fever, cough, and worsening shortness of breath. The patient's parents report that he recently recovered from prolonged diarrhea due to Giardia infection. Medical history is significant for recurrent ear infections treated with antibiotics since age 6 months and lobar pneumonia requiring hospitalization. Temperature is 38.7 C (101.7 F), pulse is 140/min, and respirations are 60/min. Physical examination reveals small tonsils and crackles in the lower lobe of the right lung. The child's weight is at the 40th percentile. Which of the following is the most likely cause of this patient's recurrent infections?

Question

A 3-year-old boy is brought to the emergency department for 2 days of fever, cough, and worsening shortness of breath.  The patient's parents report that he recently recovered from prolonged diarrhea due to Giardia infection.  Medical history is significant for recurrent ear infections treated with antibiotics since age 6 months and lobar pneumonia requiring hospitalization.  Temperature is 38.7 C (101.7 F), pulse is 140/min, and respirations are 60/min.  Physical examination reveals small tonsils and crackles in the lower lobe of the right lung.  The child's weight is at the 40th percentile.  Which of the following is the most likely cause of this patient's recurrent infections?

Quizplus · Accepted Answer

11ec25c8_d62c_31a3_bc81_6f7cb8e474dc_MD0003_00 This patient's recurrent pneumonias, Giardia infection, and small tonsils are suggestive of X-linked agammaglobulinemia (XLA), or Bruton agammaglobulinemia. XLA is caused by a genetic defect in Bruton tyrosine kinase, resulting in abnormal B-lymphocyte maturation. Low B-cell concentrations lead to small or absent lymphoid tissue (eg, tonsils, adenoids) and severely decreased serum immunoglobulin concentrations. Due to impaired humoral immunity response, infants with XLA are predisposed to recurrent sinopulmonary infections (eg, acute otitis media, pneumonia) with encapsulated organisms such as Haemophilus influenzae and Streptococcus pneumoniae. The absence of IgA also increases the risk of gastrointestinal infections (eg, Giardia). Patients usually become infected after age 3-6 months, when protection from maternally acquired IgG begins to wane. Treatment of XLA is immunoglobulin replacement therapy. Prophylactic antibiotics may be warranted if intravenous immunoglobulin therapy is unsuccessful. Live vaccines are contraindicated in XLA; other vaccines are not contraindicated but do not generate an effective antibody response. (Choice B) Adenosine deaminase deficiency is one of several gene defects that results in impaired development of B and T cells and causes severe combined immunodeficiency (SCID). Affected patients typically develop severe, recurrent viral, fungal (eg, Candida), and bacterial infections in infancy, along with failure to thrive. SCID is unlikely in a 3-year-old with normal growth. (Choice C) Patients with complement deficiencies are at increased risk for disseminated bacterial infections, particularly with encapsulated bacteria (eg, Streptococcus pneumoniae, Neisseria meningitidis). Giardia infection is not associated with complement deficiencies. (Choice D) Impaired oxidative burst occurs in chronic granulomatous disease. In contrast to this case, patients have recurrent skin and pulmonary infections with catalase-positive organisms (eg, Staphylococcus aureus, Serratia marcescens). (Choice E) Thymic hypoplasia is consistent with DiGeorge syndrome (22q11.2 microdeletion syndrome), which is characterized by hypocalcemia, cardiac defects, and failure to thrive in addition to recurrent infections. Educational objective: X-linked (or Bruton) agammaglobulinemia is characterized by severely decreased immunoglobulins due to a failure of B-lymphocyte maturation. Affected patients have small or absent lymphoid tissue and develop recurrent sinopulmonary and gastrointestinal infections once protection from transplacental maternal antibody wanes. References: X-linked agammaglobulinemia. (http://www.ncbi.nlm.nih.gov/pubmed/26909497)

A 3-Year-Old Boy Is Brought to the Emergency Department for 2