A 10-month-old boy is brought to the emergency department due to respiratory distress. He developed a fever yesterday, and his breathing has become increasingly labored. The boy was recently admitted due to streptococcal pneumonia 2 months ago, which required chest tube placement and a weeklong hospital stay. Medical history is also notable for recurrent ear infections starting at age 5 months, at which time his weight dropped from the 40th to the 20th percentile. Temperature is 39.2 C (102.6 F) , blood pressure is 100/68 mm Hg, pulse is 138/min, and respirations are 40/min. Heart sounds are normal. Lung examination reveals diffuse scattered crackles. Several collections of small, red, dilated blood vessels are visible on the bilateral sclera. There are no palpable lymph nodes. A tracheal aspirate is positive for Pneumocystis jirovecii. Which of the following laboratory results is most consistent with this patient's likely underlying diagnosis?

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Explanation: 11ec25c8_d621_3509_bc81_8d8db861ce21_MD0003_00 This patient's history of recurrent infections and failure to thrive in infancy is concerning for a primary immunodeficiency, which can involve a defect in B cells, T cells, or both. B-cell immunodeficiency: Presentation classically occurs after age 4-6 months when maternal antibodies wane. Sinopulmonary infections (eg, acute otitis media, pneumonia) caused by encapsulated bacteria are typical due to lack of opsonizing IgG and mucosal IgA. T-cell immunodeficiency: Presentation typically occurs in early infancy with severe bacterial, viral, and fungal (eg, Pneumocystis jirovecii) opportunistic infections. Failure to thrive is usually present due to chronic diarrhea and increased metabolic needs from recurrent infections. This infant's recurrent acute otitis media since age 5 months is indicative of decreased B cells and low IgA, whereas his P jirovecii pneumonia (PJP) is suggestive of decreased T cells. Therefore, a combined B- and T-cell disorder is most likely in this patient. In the presence of ocular telangiectasias (ie, collection of dilated blood vessels), his findings are most consistent with ataxia-telangiectasia (AT). AT is an autosomal recessive disorder in which defective DNA repair results in cerebellar ataxia, oculocutaneous telangiectasias, and recurrent sinopulmonary infections. Immune defects are variable and can affect both humoral and cellular immunity, as seen in this patient. Lymphoid tissue is often small/nonpalpable due to lymphopenia. ocular telangiectasias (Choices B and E) A B-cell defect (eg, X-linked agammaglobulinemia), in which B cells and immunoglobulins are decreased, or an isolated IgA deficiency would cause recurrent sinopulmonary infections but not PJP. (Choice C) Isolated T-cell defects (eg, HIV infection) present with severe, recurrent opportunistic infections (eg, PJP) and failure to thrive. Recurrent acute otitis media is more consistent with a humoral defect, and HIV infection causes generalized lymphadenopathy in infants because B cells are produced normally. (Choice D) Phagocytic (eg, chronic granulomatous disease [CGD]) and complement disorders are primary immunodeficiencies that are characterized by normal lymphocyte counts and immunoglobulin levels but defective bacterial killing. CGD is characterized by recurrent infections (eg, abscess, pneumonia) due to catalase-positive organisms (eg, Staphylococcus aureus, Aspergillus), and patients with complement disorders are predisposed to Neisseria infections. Neither condition is associated with PJP. Educational objective: B-cell defects typically present with sinopulmonary infections after age 4-6 months, whereas T-cell defects present with opportunistic infections (eg, Pneumocystis jirovecii) and failure to thrive in early infancy. Ataxia-telangiectasia is an example of a combined B- and T-cell immunodeficiency, and cerebellar ataxia and oculocutaneous telangiectasias are classic findings. References: Ataxia telangiectasia: a review. (http://www.ncbi.nlm.nih.gov/pubmed/27884168)

A 10-Month-Old Boy Is Brought to the Emergency Department Due