An occupational therapist in her late fifties is brought to the emergency department by he… — USMLE Step 2 CK

An occupational therapist in her late fifties is brought to the emergency department by her partner due to sudden-onset high fever, profound weakness, and bleeding gums over the last $48\text{ hours}$ . She has a documented history of severe, erosive rheumatoid arthritis that has been clinically silent for the past two years on a maintenance regimen of azathioprine and low-dose prednisone. She also takes sertraline for generalized anxiety disorder.

Three weeks ago, she experienced a severely painful, swollen, and erythematous right first toe. Arthrocentesis yielded fluid containing negatively birefringent, needle-shaped crystals. Following resolution of the acute episode, her rheumatologist initiated a daily prophylactic medication to prevent future articular attacks.

Upon arrival today, her temperature is $39.5^\circ\text{C}$ ( $103.1^\circ\text{F}$ ), pulse is $118\text{/min}$ , and blood pressure is $100/60\text{ mmHg}$ . Physical examination reveals marked mucosal pallor, diffuse lower extremity petechiae, and continuous oozing from the gingival margins.

Laboratory findings:
Leukocytes: $750/\text{mm}^3$
Hemoglobin: $6.8\text{ g/dL}$
Platelets: $14,000/\text{mm}^3$
Thyroid-stimulating hormone: $3.9\text{ \mu U/mL}$ (Normal: $0.4-4.2\text{ \mu U/mL}$ )
Free thyroxine (T4): $1.2\text{ ng/dL}$ (Normal: $0.8-2.8\text{ ng/dL}$ )

Which of the following mechanisms is the most likely primary cause of this patient's current hematologic crisis?

Impaired hepatic degradation of a purine analogCevap
B
Decreased hematopoietic stimulation secondary to primary thyroid failure
C
Excessive central synaptic accumulation of serotonin
D
Systemic venous congestion from acute ventricular failure
E
Bone marrow toxicity from empiric broad-spectrum antibiotic use

Cevap

Impaired hepatic degradation of a purine analog

The patient's presentation of severe neutropenia, anemia, and thrombocytopenia is highly characteristic of profound bone marrow suppression. She has a history of rheumatoid arthritis managed with azathioprine, a purine analog prodrug that is converted into 6-mercaptopurine (6-MP). The active metabolite 6-MP is subsequently degraded and inactivated by the enzyme xanthine oxidase in the liver. Her recent presentation with negatively birefringent crystals confirms a gout flare, for which she was started on prophylactic urate-lowering therapy, most likely allopurinol. Allopurinol is a potent xanthine oxidase inhibitor. When administered concurrently with azathioprine, it drastically reduces the degradation of 6-MP, leading to toxic systemic accumulation. This pharmacokinetic interaction reliably produces life-threatening pancytopenia. Therefore, the impaired hepatic degradation of the purine analog directly explains her critical hematologic crisis.

Adım Adım Çözüm

Identify the patient's underlying conditions and baseline medications.

The patient has rheumatoid arthritis managed with azathioprine and prednisone.

Establishing the baseline medication regimen is crucial for identifying potential dangerous drug interactions.

Diagnose the recent acute joint episode based on synovial fluid findings.

Negatively birefringent, needle-shaped crystals confirm an acute gout flare.

Accurate diagnosis of the interval event determines the likely new medication added (urate-lowering therapy).

Determine the mechanism of the prophylactic medication prescribed for the interval event.

Allopurinol, a xanthine oxidase inhibitor, is the standard first-line urate-lowering therapy to prevent future gout attacks.

Identifying the mechanism of the new drug is required to evaluate its interaction with the baseline regimen.

Evaluate the pharmacokinetic interaction between the baseline and new medications.

Allopurinol profoundly inhibits xanthine oxidase, the enzyme responsible for degrading 6-mercaptopurine (the active metabolite of azathioprine).

This specific enzymatic inhibition leads to toxic accumulation of the active purine analog.

Correlate the pathophysiologic mechanism with the current clinical presentation.

Toxic levels of 6-mercaptopurine cause profound bone marrow suppression, manifesting as the patient's severe pancytopenia, fever, and mucosal bleeding.

This confirms that impaired hepatic degradation of a purine analog is the definitive cause of the crisis.

Anahtar Kavram

Pharmacokinetic interaction between azathioprine and xanthine oxidase inhibitors

Tahmini Süre:1m 30s

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