A 58-year-old woman presents to the outpatient clinic reporting a 2-month history of worse… — USMLE Step 3

A $58$ -year-old woman presents to the outpatient clinic reporting a $2$ -month history of worsening fatigue, diffuse joint pain, and a new rash on her lower legs. She has a past medical history of hypertension and underwent a cholecystectomy in $1989$ that required a blood transfusion. Her current medications include amlodipine. Vital signs show a temperature of $37.3^\circ\text{C}$ ( $99.1^\circ\text{F}$ ), blood pressure of $148/92\text{ mmHg}$ , pulse of $84\text{/min}$ , and respiratory rate of $14\text{/min}$ . Physical examination reveals non-blanching, palpable purpuric lesions across her bilateral distal lower extremities. The metacarpophalangeal joints are mildly swollen and tender bilaterally.

Laboratory evaluation reveals the following:

Laboratory Test	Result
Hemoglobin	$11.2 \text{ g/dL}$
Platelet count	$215,000/\mu\text{L}$
Leukocyte count	$7,200/\mu\text{L}$
Serum creatinine	$1.9 \text{ mg/dL}$ (baseline $0.9 \text{ mg/dL}$ )
Rheumatoid factor	Positive
Complement C3	Normal
Complement C4	Markedly decreased

Urinalysis demonstrates $3+$ protein, $25-30$ red blood cells/hpf, and the presence of red blood cell casts.

Which of the following is the most likely diagnosis?

A
IgA vasculitis
B
Disseminated intravascular coagulation
Mixed cryoglobulinemia syndromeAnswer
D
Thrombotic thrombocytopenic purpura
E
Rheumatoid arthritis

Answer

Mixed cryoglobulinemia syndrome

The correct diagnosis is mixed cryoglobulinemia syndrome. This patient presents with the classic triad of palpable purpura, arthralgias, and fatigue, accompanied by immune complex-mediated glomerulonephritis (indicated by hypertension, elevated creatinine, and RBC casts). Mixed cryoglobulinemia is overwhelmingly associated with chronic Hepatitis C virus (HCV) infection. Her history of a blood transfusion in 1989—prior to the implementation of routine HCV screening of the blood supply in 1992—places her at high risk for chronic HCV. The laboratory profile is highly characteristic: the cryoglobulins in this condition are typically IgM antibodies with rheumatoid factor (RF) activity, explaining the positive RF test. The resultant immune complexes robustly activate the classical complement pathway, leading to a hallmark laboratory finding: markedly decreased C4 complement levels with relatively preserved C3 levels.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient has the classic "Meltzer triad" of palpable purpura, arthralgias, and systemic symptoms (fatigue), combined with evidence of acute nephritic syndrome (elevated creatinine, proteinuria, hematuria, RBC casts).

Identifying the core syndrome narrows the differential diagnosis to small-vessel vasculitides and immune complex-mediated diseases.

Evaluate the patient's medical history for risk factors.

The patient received a blood transfusion in 1989, which is a major risk factor for acquiring Hepatitis C virus (HCV), as widespread blood screening for HCV was not implemented until 1992.

Chronic HCV infection is the primary driver of secondary mixed cryoglobulinemia syndrome.

Interpret the specific laboratory findings to finalize the diagnosis.

The labs show a positive rheumatoid factor (RF), a markedly decreased C4 level, and a normal C3 level.

Cryoglobulins are often IgM autoantibodies with RF activity (binding to the Fc portion of IgG). This immune complex formation robustly activates the classical complement pathway, leading to a profound, isolated depletion of C4, which is highly specific for mixed cryoglobulinemia.

Key Concept

Formulating a differential diagnosis for palpable purpura and glomerulonephritis by integrating historical risk factors (HCV exposure) with specific serologic findings (positive RF, profoundly low C4).

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic