An odd case of pallor and splenomegaly

THE CASE

You are completing rounds as an intern one morning on the general pediatric ward when your colleague from the emergency department (ED) signs out a patient to you. She describes an 11-month-old black male who presented with a 4-day history of low-grade fever, nasal congestion, and decreased appetite, without vomiting or diarrhea.

In the ED, he is transfused with packed red blood cells (PRBCs) at 10 mL/kg because of his hemoglobin level of 7 gm/dL (normal for an 11-month-old is 12 g/dL, with 10.5 g/dL being 2 [standard deviation] below the mean) and accompanying symptoms of pallor, fatigue, irritability, and tachycardia (162 beats per minute [bpm]; mean for age is 120 bpm, with normal range of 90 bpm-150 bpm).

He was born full term, normal vaginal delivery, at a birth weight of 7 lbs and was breastfed as an infant and subsequently introduced to table food with good tolerance. He has not yet been exposed to cow's milk. He had no prior admissions to the hospital and has not had any surgeries.

Based on your review of the literature, you decide to proceed with some basic laboratory tests. The complete blood count (performed on blood obtained before transfusion) reveals a white blood cell (WBC) count of 2.1 K/μL; hemoglobin, 7 g/dL; and platelet count, 34,000/μL. The reticulocyte count is depressed at 0.1% (normal, 0.5%-1%), but the red blood cells (RBCs) appear normocytic (mean cell volume [MCV]=76 fL; normal range, 72 fL-79 fL) but hypochromic, with slight polychromasia.

The differential reveals a low absolute neutrophil count of 0.36 K/μL (1.70-7.30), indicating that he is severely neutropenic. The remainder of the differential is normal. His metabolic panel reveals a sodium level of 138 mEq/L (normal, 133 mEq/L-146 mEq/L); potassium of 3.1 mEq/L (normal, 3.4 mEq/L-4.7 mEq/L); and magnesium of 1.3 mg/dL (normal, 1.3 mg/dL-2 mg/dL). The blood urea nitrogen (BUN), creatinine, chloride, bicarbonate, aspartate aminotransferase (AST), alanine aminotransferase (ALT), haptoglobin, and coagulation profile values are within normal ranges. A uric acid level ordered from the ED returns high at 8 mg/dL (normal, 2.4 mg/dL-5.9 mg/dL).

Worried about the hyperuricemia, you decide to hydrate the boy with intravenous (IV) fluids containing bicarbonate but cautiously avoid potassium because you are concerned about tumor lysis syndrome. You consult the hematology attending, who advises you to add a dose of rasburicase, one of the newer agents to treat hyperuricemia and tumor lysis syndrome. He tells you that it is a recombinant urate oxidase that helps to catalyze uric acid to a far more soluble metabolite called allantoin and thus reduces the risk of renal failure.