A 48-year-old woman is evaluated in the clinic for a 4-month history of intermittent joint… — USMLE Step 3

A $48$ -year-old woman is evaluated in the clinic for a $4$ -month history of intermittent joint pain in her hands and knees, severe fatigue, and a recurrent red rash on her legs. She has no significant past medical history and takes no medications. Vital signs are within normal limits. Physical examination reveals non-blanching, purpuric papules on both lower extremities. Laboratory evaluation shows:

Laboratory Test	Result	Reference Range
Hemoglobin	$11.2$ g/dL	$12.0$ - $16.0$ g/dL
Platelets	$190,000/\text{mm}^3$	$150,000$ - $400,000/\text{mm}^3$
Leukocytes	$6,500/\text{mm}^3$	$4,500$ - $11,000/\text{mm}^3$
Creatinine	$1.6$ mg/dL	$0.6$ - $1.2$ mg/dL
Aspartate aminotransferase (AST)	$65$ U/L	$8$ - $20$ U/L
Alanine aminotransferase (ALT)	$72$ U/L	$8$ - $20$ U/L
Antinuclear antibody (ANA)	Negative	Negative
Rheumatoid factor	$320$ IU/mL	$<15$ IU/mL
Complement C3	$110$ mg/dL	$80$ - $160$ mg/dL
Complement C4	$5$ mg/dL	$16$ - $48$ mg/dL

Urinalysis reveals 2+ protein, $15$ - $20$ red blood cells/hpf, and no cellular casts.

Based on the interpretation of this patient's laboratory profile, which of the following is the most likely diagnosis?

Hepatitis C-associated mixed cryoglobulinemiaAnswer
B
Systemic lupus erythematosus
C
Post-streptococcal glomerulonephritis
D
Microscopic polyangiitis
E
Hepatitis B-associated polyarteritis nodosa

Answer

The correct diagnosis is Hepatitis C-associated mixed cryoglobulinemia.

The correct answer is Hepatitis C-associated mixed cryoglobulinemia. This patient presents with the classic Meltzer triad (palpable purpura, arthralgias, fatigue) along with glomerulonephritis and mild transaminitis. The critical diagnostic step is the interpretation of her complement panel. She has a normal C3 level but a severely depressed C4 level, paired with a highly elevated rheumatoid factor. In Type II mixed cryoglobulinemia, a monoclonal IgM (acting as a rheumatoid factor) binds to IgG to form immune complexes. These complexes robustly activate the classical complement pathway, massively consuming early components like C4. However, the enzymatic cascade is relatively inefficient at the C3 convertase step in this disease, leaving C3 levels normal or only slightly reduced. This specific C4-dominant consumption pattern, coupled with high rheumatoid factor and hepatic inflammation, is essentially pathognomonic for HCV-associated mixed cryoglobulinemia.

Step-by-Step Solution

Analyze the clinical presentation.

The patient exhibits the 'Meltzer triad' of palpable purpura, arthralgias, and weakness, accompanied by evidence of new-onset glomerulonephritis (elevated creatinine, proteinuria, hematuria).

Identifying the clinical syndrome narrows the differential to systemic vasculitides and connective tissue diseases.

Interpret the immunologic laboratory results, specifically the complement profile.

The patient has a markedly low C4 level (

5

mg/dL) with a normal C3 level (

110

mg/dL), alongside a significantly elevated rheumatoid factor.

The dissociation between C3 and C4 is the critical diagnostic pivot point. Isolated profound C4 depression with normal C3 indicates specific classical pathway activation where the C3 convertase is inefficient, characteristic of cryoglobulinemia.

Correlate the laboratory findings with the pathophysiology of the suspected disease.

In Type II mixed cryoglobulinemia, monoclonal IgM with rheumatoid factor activity binds to polyclonal IgG. These immune complexes rapidly consume early complement components (C1, C2, C4) but often spare C3.

This confirms the diagnosis of mixed cryoglobulinemia, which is overwhelmingly associated with Hepatitis C virus (supported by the mild transaminitis).

Key Concept

The hallmark laboratory finding of mixed cryoglobulinemia is a severely depressed C4 level with a normal or only mildly depressed C3 level, accompanied by a high positive rheumatoid factor.

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