An oncology care team is preparing to initiate induction chemotherapy for a recently admit… — USMLE Step 3

An oncology care team is preparing to initiate induction chemotherapy for a recently admitted patient.

Demographics: $19$ -year-old male of Mediterranean descent
Vital Signs: Temperature $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), HR $98\text{/min}$ , BP $118/74\text{ mmHg}$ , RR $16\text{/min}$
Physical Exam: Noticeably enlarged, non-tender submandibular and cervical lymph nodes. No hepatosplenomegaly.
Laboratory Findings:
- WBC: $12,500\text{/mm}^3$
- Hemoglobin: $13.8\text{ g/dL}$
- Platelets: $180,000\text{/mm}^3$
- Uric acid: $11.2\text{ mg/dL}$ (Normal: $4.0-8.5\text{ mg/dL}$ )
- LDH: $950\text{ U/L}$ (Normal: $140-280\text{ U/L}$ )
- Creatinine: $1.1\text{ mg/dL}$

A peripheral blood smear demonstrates numerous medium-sized atypical lymphocytes with vacuolated, deeply basophilic cytoplasm. Cytogenetic analysis confirms a $t(8;14)$ translocation. Given the high proliferation index and elevated baseline uric acid, the patient is at significant risk for an oncologic emergency characterized by massive release of intracellular metabolites. The attending physician recommends administering a recombinant enzyme that converts a relatively insoluble purine degradation product into a highly soluble compound. However, before administration, the physician orders an erythrocyte enzyme assay; if this assay demonstrates a specific underlying deficiency, the recombinant agent is absolutely contraindicated.

Which of the following best explains the pathophysiologic mechanism underlying this absolute contraindication?

A
The underlying deficiency prevents the conversion of the recombinant enzyme into its active form, necessitating alternative prophylactic measures.
B
The recombinant enzyme strongly inhibits cytochrome P450, leading to fatal toxicity when combined with the induction chemotherapy regimen.
The recombinant enzyme generates hydrogen peroxide as a byproduct, precipitating severe oxidative erythrocyte damage.Cevap
D
The underlying deficiency impairs hepatic glucuronidation, causing toxic accumulation of the drug and its metabolites.
E
The recombinant enzyme rapidly depletes circulating calcium levels, exacerbating the pre-existing hypocalcemia associated with this oncologic emergency.

Cevap

The correct answer states that the recombinant enzyme generates hydrogen peroxide as a byproduct, precipitating severe oxidative erythrocyte damage.

The patient's clinical presentation (rapidly enlarging nodes, vacuolated basophilic lymphocytes, and a

t(8;14)

translocation) is classic for Burkitt lymphoma. The high proliferation rate puts the patient at significant risk for tumor lysis syndrome (TLS). Rasburicase, a recombinant urate oxidase, is used to prevent TLS by catalyzing the oxidation of relatively insoluble uric acid to the highly soluble compound allantoin. However, this reaction produces hydrogen peroxide (

H_2O_2

) as a toxic byproduct. In patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, erythrocytes cannot generate sufficient NADPH to reduce glutathione, leaving them unable to neutralize oxidative stressors like

H_2O_2

. Consequently, administration of rasburicase in G6PD-deficient patients (who are more common in Mediterranean populations) causes severe oxidative erythrocyte damage, leading to massive hemolysis and methemoglobinemia. Therefore, rasburicase is strictly contraindicated in this population.

Adım Adım Çözüm

Identify the underlying diagnosis and complication risk based on the clinical, morphological, and cytogenetic findings.

The patient has Burkitt lymphoma (

t(8;14)

, basophilic vacuolated lymphocytes) with a high risk of developing Tumor Lysis Syndrome (TLS) due to a high proliferation index.

Establishing the diagnosis guides the necessary prophylactic interventions required before initiating cytotoxic chemotherapy.

Identify the recommended prophylactic agent described in the vignette.

The agent that converts uric acid to a highly soluble compound is rasburicase, a recombinant urate oxidase.

Identifying the specific drug is necessary to evaluate its adverse effect profile and contraindications.

Recognize the prerequisite screening test based on the patient's demographic information.

Screening for glucose-6-phosphate dehydrogenase (G6PD) deficiency is required, as the patient is of Mediterranean descent and is at risk for a severe variant of the deficiency.

This step connects the demographic clue to the required clinical assay.

Correlate the drug's mechanism of action with the underlying deficiency.

Rasburicase produces hydrogen peroxide (

H_2O_2

). G6PD deficiency impairs NADPH production, preventing the neutralization of

H_2O_2

by glutathione peroxidase, resulting in severe hemolysis.

This establishes the exact pathophysiologic mechanism of the absolute contraindication.

Anahtar Kavram

Rasburicase is a recombinant urate oxidase used to prevent tumor lysis syndrome by converting uric acid to allantoin. Because it generates hydrogen peroxide as a byproduct, it is absolutely contraindicated in patients with G6PD deficiency due to the risk of severe oxidative erythrocyte damage (hemolysis and methemoglobinemia).

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