A 19-year-old male of Mediterranean descent is admitted to the hospital with newly diagnos… — USMLE Step 2 CK

A $19$ -year-old male of Mediterranean descent is admitted to the hospital with newly diagnosed Burkitt lymphoma. He has a massive abdominal tumor burden. Baseline laboratory studies reveal a serum uric acid level of $14.2 \text{ mg/dL}$ and a serum creatinine of $1.9 \text{ mg/dL}$ . To mitigate the risk of tumor lysis syndrome, he receives aggressive intravenous hydration and a dose of rasburicase. Forty-eight hours later, the patient acutely develops shortness of breath, fatigue, and bluish discoloration of his lips and fingertips. Vital signs are: temperature $37.1^\circ \text{C}$ , heart rate $128$ /min, blood pressure $105/65$ mmHg, and respiratory rate $28$ /min. Pulse oximetry shows an oxygen saturation of $84\%$ on room air, which does not improve following the administration of $100\%$ oxygen via a non-rebreather mask. Arterial blood gas on $100\%$ oxygen shows a PaO $_2$ of $410$ mmHg. Blood drawn from the patient's vein appears chocolate-brown. Laboratory studies show a rapid decline in hemoglobin from $14.5 \text{ g/dL}$ to $8.2 \text{ g/dL}$ . A peripheral blood smear reveals numerous bite cells and Heinz bodies. Which of the following is the most appropriate next step in the management of this patient's acute respiratory compromise?

Perform red blood cell exchange transfusionCevap
B
Administer intravenous methylene blue
C
Administer intravenous ascorbic acid
D
Initiate therapeutic enoxaparin and order a CT pulmonary angiography
E
Transfuse two units of packed red blood cells

Cevap

Perform red blood cell exchange transfusion

The correct answer is to perform a red blood cell exchange transfusion. This patient has developed acute severe methemoglobinemia and hemolytic anemia secondary to rasburicase administration. Rasburicase is a recombinant urate oxidase used to prevent tumor lysis syndrome by converting uric acid to allantoin. This reaction generates large amounts of hydrogen peroxide (

H_2O_2

) as a byproduct. In patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency (suggested by his Mediterranean descent and the presence of bite cells and Heinz bodies on the blood smear), the inability to generate NADPH renders erythrocytes highly susceptible to oxidative stress. This stress simultaneously causes hemolysis and oxidizes normal ferrous hemoglobin (Fe

^{2+}

) to ferric methemoglobin (Fe

^{3+}

). Methemoglobinemia is characterized by central cyanosis, a 'saturation gap' (low pulse oximetry reading despite a high PaO

_2

on arterial blood gas), and chocolate-brown blood. While methylene blue is the standard first-line treatment for methemoglobinemia, its mechanism of action requires NADPH from the G6PD pathway. In G6PD-deficient patients, methylene blue is not only ineffective but also acts as a potent exogenous oxidant, precipitating a potentially fatal hemolytic crisis. Therefore, severe symptomatic methemoglobinemia in a G6PD-deficient patient must be managed with a red blood cell exchange transfusion (or hyperbaric oxygen therapy) to physically remove the methemoglobin and replace it with functional red blood cells.

Adım Adım Çözüm

Analyze the patient's history and peripheral blood smear.

The presence of bite cells, Heinz bodies, and Mediterranean descent in the context of an oxidative trigger confirms underlying glucose-6-phosphate dehydrogenase (G6PD) deficiency.

Identifying this underlying enzyme deficiency is critical, as it fundamentally alters the safety profile of subsequent therapeutic interventions.

Interpret the oxygenation paradox and distinct clinical signs.

A persistently low pulse oximetry reading despite a very high PaO2 (a 'saturation gap'), accompanied by central cyanosis and chocolate-brown blood, confirms the diagnosis of severe methemoglobinemia.

Rasburicase (a recombinant urate oxidase) catalyzed the breakdown of uric acid into allantoin and hydrogen peroxide. The accumulated hydrogen peroxide caused massive oxidative stress, leading to both hemolysis and the oxidation of hemoglobin to methemoglobin.

Select the appropriate therapy for methemoglobinemia based on the patient's G6PD status.

Red blood cell exchange transfusion is chosen as the definitive intervention.

The standard first-line treatment, methylene blue, requires the G6PD-dependent production of NADPH to reduce methemoglobin. In G6PD deficiency, methylene blue is ineffective and acts as a dangerous oxidant, while exchange transfusion safely removes the defective erythrocytes.

Anahtar Kavram

Patients with G6PD deficiency who develop severe, symptomatic methemoglobinemia (e.g., secondary to rasburicase-induced oxidative stress) must be treated with exchange transfusion or hyperbaric oxygen, as methylene blue is strictly contraindicated.

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