A 9-year-old boy is admitted to the pediatric oncology unit with newly diagnosed T-cell ac… — USMLE Step 2 CK

A 9-year-old boy is admitted to the pediatric oncology unit with newly diagnosed T-cell acute lymphoblastic leukemia. He has a 2-week history of worsening fatigue, bone pain, and shortness of breath. Chest radiograph shows a large anterior mediastinal mass. Before the initiation of induction chemotherapy, laboratory studies are obtained:

Laboratory Test	Result	Reference Range
Leukocyte count	$165,000/\text{mm}^3$	$4,500-11,000/\text{mm}^3$
Hemoglobin	$8.4\text{ g/dL}$	$11.5-15.5\text{ g/dL}$
Platelet count	$48,000/\text{mm}^3$	$150,000-400,000/\text{mm}^3$
Blood urea nitrogen	$38\text{ mg/dL}$	$7-20\text{ mg/dL}$
Creatinine	$1.6\text{ mg/dL}$	$0.3-0.7\text{ mg/dL}$
Uric acid	$12.4\text{ mg/dL}$	$2.0-5.5\text{ mg/dL}$
Calcium	$7.4\text{ mg/dL}$	$8.8-10.8\text{ mg/dL}$
Phosphorus	$7.2\text{ mg/dL}$	$3.0-5.4\text{ mg/dL}$
Potassium	$4.8\text{ mEq/L}$	$3.4-4.7\text{ mEq/L}$
Lactate dehydrogenase	$5,200\text{ U/L}$	$120-300\text{ U/L}$

An electrocardiogram shows normal sinus rhythm with no ST-segment or T-wave abnormalities. The patient does not exhibit tetany or seizures. Vigorous intravenous hydration with normal saline is initiated.

Which of the following is the most appropriate additional pharmacologic intervention for this patient's metabolic derangements?

Intravenous rasburicaseAnswer
B
Intravenous allopurinol
C
Intravenous sodium bicarbonate
D
Intravenous calcium gluconate
E
Oral sevelamer

Answer

The most appropriate intervention is to administer intravenous rasburicase to rapidly degrade the severely elevated uric acid.

The correct answer is intravenous rasburicase. This patient presents with spontaneous tumor lysis syndrome (TLS) secondary to T-cell acute lymphoblastic leukemia. TLS is characterized by the massive release of intracellular contents, leading to hyperkalemia, hyperphosphatemia, hyperuricemia, and secondary hypocalcemia. This patient has extremely high-risk features (leukocyte count

>100,000/\text{mm}^3

and significantly elevated LDH) and has already developed acute kidney injury (AKI) from uric acid nephropathy (creatinine

1.6\text{ mg/dL}

, uric acid

12.4\text{ mg/dL}

). In this scenario, aggressive intravenous hydration combined with rasburicase is the standard of care. Rasburicase is a recombinant urate oxidase enzyme that rapidly catalyzes the conversion of existing poorly soluble uric acid into allantoin, an inactive and highly water-soluble metabolite that is easily excreted. This rapid clearance of uric acid is necessary to reverse the AKI.

Step-by-Step Solution

Identify the clinical syndrome

The patient has spontaneous tumor lysis syndrome (TLS).

High tumor burden (WBC 165,000/mm³, huge mediastinal mass, LDH 5,200 U/L) has led to the release of intracellular contents, resulting in hyperuricemia, hyperphosphatemia, and secondary hypocalcemia prior to treatment.

Assess the severity and organ function

The patient has established acute kidney injury (creatinine 1.6 mg/dL).

The severe uric acid level of 12.4 mg/dL has likely caused urate crystal precipitation in the renal tubules, leading to an acute decline in kidney function.

Select the appropriate pharmacologic therapy

Intravenous rasburicase is required.

Because the patient already has severe hyperuricemia and AKI, preventing new uric acid formation (with allopurinol) is insufficient. Rasburicase degrades pre-existing uric acid into highly soluble allantoin, rapidly clearing it from the serum and protecting the kidneys.

Key Concept

In patients with high-risk tumor lysis syndrome and established hyperuricemia or acute kidney injury, rasburicase is indicated over allopurinol because it rapidly degrades existing uric acid into soluble allantoin.

Estimated Time:1m 30s

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