A 44-year-old man is evaluated in the hospital for progressive agitation and confusion. He… — USMLE Step 2 CK

A $44$ -year-old man is evaluated in the hospital for progressive agitation and confusion. He was admitted two days ago for a severe right lower extremity cellulitis caused by methicillin-resistant Staphylococcus aureus (MRSA) and was started on intravenous linezolid. He has a history of major depressive disorder and generalized anxiety disorder, for which he takes multiple stable home medications. His temperature is $39.1^\circ\text{C}$ ( $102.4^\circ\text{F}$ ), blood pressure is $168/102$ mmHg, pulse is $124$ /min, and respirations are $22$ /min. Physical examination reveals profuse diaphoresis, bilateral mydriasis, and a resting tremor. Neurological examination is notable for pronounced hyperreflexia in the lower extremities and sustained ankle clonus. A dangerous interaction with which of the following of the patient's home medications is the most likely cause of this presentation?

FluoxetineAnswer
B
Haloperidol
C
Bupropion
D
Aripiprazole
E
Lorazepam

Answer

Fluoxetine

This patient's presentation of altered mental status, autonomic instability (hyperthermia, tachycardia, hypertension, diaphoresis), and prominent neuromuscular hyperactivity (hyperreflexia and clonus) is classic for serotonin syndrome. The syndrome was precipitated by the administration of linezolid. Linezolid is an antibiotic used to treat resistant Gram-positive infections (like MRSA) that also acts as a reversible, non-selective monoamine oxidase inhibitor (MAOI). When combined with a serotonergic agent like an SSRI (e.g., fluoxetine), it blocks the breakdown of serotonin, leading to massive synaptic accumulation and toxicity. Clinicians must carefully review psychiatric medication lists before initiating linezolid therapy.

Step-by-Step Solution

Evaluate the clinical presentation.

The patient has altered mental status, autonomic instability (fever, tachycardia, hypertension, diaphoresis), and neuromuscular hyperactivity (hyperreflexia, sustained clonus).

Identifying the clinical triad points directly to a diagnosis of serotonin syndrome.

Analyze the patient's recent medication changes.

The patient was recently started on intravenous linezolid for an MRSA infection.

Linezolid is an oxazolidinone antibiotic that also possesses reversible, non-selective monoamine oxidase inhibitor (MAOI) properties.

Identify the contraindicated home medication.

Combining an MAOI (linezolid) with a selective serotonin reuptake inhibitor (SSRI) like fluoxetine creates a dangerous drug-drug interaction.

The concurrent use of these medications leads to impaired serotonin degradation and increased serotonin release, resulting in synaptic serotonin toxicity (serotonin syndrome).

Key Concept

Serotonin syndrome can be precipitated by combining a selective serotonin reuptake inhibitor (SSRI) with linezolid, an antibiotic that possesses monoamine oxidase inhibitor (MAOI) activity.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic