A 62-year-old man comes to the primary care clinic reporting severe, diffuse muscle aches,… — USMLE Step 3

A 62-year-old man comes to the primary care clinic reporting severe, diffuse muscle aches, profound generalized weakness, and passing dark brown urine for the past 48 hours. He has a past medical history of hyperlipidemia, essential hypertension, and paroxysmal atrial fibrillation. His surgical history is notable for an appendectomy in childhood. He consumes 1 to 2 glasses of wine per week and denies tobacco or illicit drug use. His chronic daily medications include atorvastatin, amlodipine, and metoprolol. Five days ago, he was evaluated at an urgent care center for an acute exacerbation of chronic bronchitis and was prescribed a 7-day course of an oral medication. Vital signs today are temperature $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure $138/86\text{ mm Hg}$ , pulse $82\text{/min}$ , and respirations $16\text{/min}$ . Laboratory studies reveal a serum creatine kinase level of $14,500\text{ U/L}$ and a serum creatinine of $2.1\text{ mg/dL}$ (baseline $1.0\text{ mg/dL}$ ). Which of the following elements of his medical and dietary history are most critical to elucidate the etiology of his current presentation? (Select all that apply)

The exact pharmacological class of the medication prescribed at the urgent care centerAnswer
Recent dietary modifications, specifically the regular consumption of grapefruit juiceAnswer
C
Current status regarding age-appropriate pneumococcal and influenza vaccinations
D
A detailed family history of adverse reactions to general anesthetics, such as malignant hyperthermia
E
Recent dosage adjustments or changes in the administration schedule of his metoprolol

Answer

The most critical historical elements are the exact pharmacological class of the urgent care medication and any recent consumption of grapefruit juice.

The correct options identify factors that inhibit the cytochrome P450 3A4 (CYP3A4) enzyme. The patient is presenting with acute rhabdomyolysis and acute kidney injury while taking atorvastatin, a CYP3A4 substrate. Co-administration of CYP3A4 inhibitors, such as macrolide antibiotics (often prescribed for bronchitis) or large quantities of grapefruit juice, can significantly decrease statin metabolism. This leads to toxic statin accumulation and subsequent severe myopathy. Therefore, eliciting the exact medication prescribed at the urgent care center and any recent dietary changes involving grapefruit are critical steps in evaluating this patient's medical history.

Step-by-Step Solution

Identify the patient's acute clinical syndrome based on presenting symptoms and laboratory findings.

The patient has rhabdomyolysis, evidenced by severe muscle aches, weakness, dark brown urine (myoglobinuria), and a markedly elevated serum creatine kinase level, complicated by acute kidney injury.

Establishing the primary diagnosis is required before determining the underlying etiology.

Review the patient's chronic medication list for potential causes or interactions.

The patient is taking atorvastatin, which is known to carry a risk of myopathy and rhabdomyolysis, particularly when its metabolism is inhibited.

Statins are a common pharmacological cause of acute muscle toxicity.

Identify potential triggers for statin toxicity in the recent medical and dietary history.

Atorvastatin is metabolized by the cytochrome P450 3A4 (CYP3A4) enzyme. Recent prescription of a macrolide antibiotic (e.g., for bronchitis) or consumption of grapefruit juice can inhibit CYP3A4, leading to toxic atorvastatin levels.

Identifying the specific CYP3A4 inhibitor is essential for definitive diagnosis and preventing future episodes.

Key Concept

Identifying drug-drug and drug-food interactions in the medical history is crucial for diagnosing acute pharmacological toxicities, such as statin-induced rhabdomyolysis triggered by CYP3A4 inhibitors.

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