A 12-month-old boy is evaluated for an eczematous rash. Medical history is significant for several severe respiratory infections that required hospitalization. Complete blood count shows white blood cells at 9,000/mm³ and platelets at 40,000/mm³. The platelets seem abnormally small and deformed on the peripheral blood smear. Which of the following is the most likely diagnosis?

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11ec160b_37c4_ac74_9249_2b1e699d5d8b_MD0001_00 This patient has the 3 characteristic findings of Wiskott-Aldrich syndrome (WAS): eczema, recurrent infections, and thrombocytopenia. Because it is an X-linked recessive disorder, WAS manifests almost exclusively in males. It is caused by a mutation in cytoskeleton proteins that are critical for normal cell structure and the cell-to-cell interactions required for activation of hematologic cells (eg, platelets, immune cells). In patients with WAS, platelets are both abnormally shaped and deficient, causing petechiae, purpura, hematemesis, and epistaxis that can be present at birth. A deficiency in B cells results in recurrent pyogenic infections due to an inability to mount a humoral immune response against organisms with a polysaccharide capsule (eg, Neisseria meningitidis, Haemophilus influenzae, Streptococcus pneumoniae). Patients also have T-cell deficiency, leading to infections with opportunistic pathogens (eg, Pneumocystis jiroveci). Severe recurrent infections develop after transplacental maternal IgG and maternal mucosal IgA are degraded around 6 months of age. Treatment is with HLA-matched bone marrow transplantation. (Choice A) Aplastic anemia (ie, pancytopenia) classically results from ionizing radiation or the use of cytotoxic medications (eg, chemotherapeutic agents, chloramphenicol). Although aplastic anemia is associated with recurrent infections, abnormally shaped platelets and eczema are more characteristic of WAS. (Choice B) Ataxia telangiectasia syndrome is associated with a deficiency of both B and T cells; however, patients characteristically have progressive ataxia and telangiectasia rather than eczema. (Choice C) Patients with Chédiak-Higashi syndrome have coagulation defects and recurrent pyogenic infections due to the dysfunction of phagocyte phagosome-lysosome fusion. However, they also typically have oculocutaneous albinism, peripheral neuropathy, and giant cytoplasmic granules on peripheral blood smear. (Choice D) DiGeorge syndrome results from maldevelopment of the third and fourth pharyngeal pouches, which causes hypoplasia of the parathyroid glands and thymus (T cell deficiency), cardiac and aortic arch abnormalities, and characteristic facies due to maldevelopment of the mandible. (Choice E) Hemolytic uremic syndrome most often occurs in children after infection with Shiga toxin-producing Escherichia coli. Patients typically have thrombocytopenia with microangiopathic hemolytic anemia and acute renal failure. Educational objective: Wiskott-Aldrich syndrome is characterized by the triad of eczema, thrombocytopenia, and combined B and T cell deficiency. Patients can have thrombocytopenia at birth, with eczema and repeated infections due to encapsulated and/or opportunistic organisms usually developing later around 6 to 12 months of age.

A 12-Month-Old Boy Is Evaluated for an Eczematous Rash