A retired 68-year-old teacher presents to the emergency department reporting two days of p… — USMLE Step 3

A retired $68$ -year-old teacher presents to the emergency department reporting two days of profound fatigue and noticeably reduced urine volume. Four days ago, he underwent an urgent percutaneous coronary intervention with stent placement for a non-ST-segment elevation myocardial infarction. He was discharged the following day in stable condition. His medical history is notable for hypertension, type 2 diabetes mellitus, and hyperlipidemia. Current medications are aspirin, clopidogrel, atorvastatin, and metoprolol.

His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $158/92\text{ mmHg}$ , and pulse is $84/\text{min}$ . Physical examination reveals a lacy, purplish discoloration in a reticular pattern over his bilateral thighs and lower legs that does not blanch with pressure. The toes on his right foot appear cyanotic, although bilateral dorsalis pedis and posterior tibial pulses are strong and symmetric.

Laboratory studies show:
- Hemoglobin: $13.2\text{ g/dL}$
- Platelet count: $210,000/\mu\text{L}$
- Leukocyte count: $10,500/\mu\text{L}$ (Neutrophils $65\%$ , Lymphocytes $25\%$ , Eosinophils $10\%$ )
- Blood urea nitrogen: $48\text{ mg/dL}$
- Serum creatinine: $2.9\text{ mg/dL}$ (baseline prior to the procedure was $1.1\text{ mg/dL}$ )
- Urinalysis: $1-2\text{ RBCs/hpf}$ , trace protein, and positive for urinary eosinophils.

Based on these clinical and laboratory findings, which of the following is the most likely diagnosis?

A
Acute interstitial nephritis
Cholesterol atheroembolismAnswer
C
Contrast-induced nephropathy
D
Heparin-induced thrombocytopenia
E
IgA vasculitis

Answer

Cholesterol atheroembolism

The correct diagnosis is cholesterol atheroembolism (also known as cholesterol crystal embolization). During invasive vascular procedures like cardiac catheterization, mechanical trauma can scrape atherosclerotic plaques off the aortic wall. These cholesterol crystals shower downstream into small arterioles, causing microvascular occlusion and an intense inflammatory reaction. The classic presentation includes acute kidney injury (often developing days to weeks after the procedure, later than contrast nephropathy), livedo reticularis, and "blue toe syndrome" where toes become cyanotic despite intact palpable distal pulses (as the occlusion is microvascular). The inflammatory response to the crystals frequently triggers both peripheral eosinophilia and eosinophiluria, which often misleads clinicians toward acute interstitial nephritis.

Step-by-Step Solution

Analyze the timeline and triggering event.

The patient developed acute kidney injury and systemic symptoms

4

days after an invasive vascular procedure (percutaneous coronary intervention).

Establishing the temporal relationship between a procedure and symptom onset is the first step in formulating a differential diagnosis for hospital-associated complications.

Evaluate the physical examination findings.

The patient has livedo reticularis (lacy, purplish rash) and blue toes, but importantly, his distal pedal pulses are strong and intact.

Intact pulses in the setting of distal ischemia strongly suggest microvascular occlusion (capillaries/arterioles) rather than a macrovascular thrombosis or saddle embolus.

Interpret the laboratory data in context.

The patient has worsening acute kidney injury accompanied by peripheral eosinophilia (

10\%

) and urinary eosinophils.

While eosinophiluria is classically taught as a marker for acute interstitial nephritis, it is also a well-documented hallmark of the inflammatory response to cholesterol crystals in atheroembolic disease.

Synthesize findings to isolate the most likely diagnosis.

The combination of recent vascular instrumentation, delayed-onset acute kidney injury, livedo reticularis, blue toes with intact pulses, and eosinophilia definitively points to cholesterol atheroembolism.

No other differential diagnosis adequately explains both the microvascular skin findings and the specific allergic-type inflammatory laboratory profile following catheterization.

Key Concept

Cholesterol atheroembolism is a multisystem complication of vascular procedures characterized by delayed acute kidney injury, livedo reticularis, blue toe syndrome with intact pulses, and a prominent eosinophilic inflammatory response.

Estimated Time:1m 30s

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