Examkin

Question

Difficulty: HardRheumatology and Immunology

A 54-year-old man presents to the clinic with a 3-month history of profound fatigue, intermittent joint pain in his hands, and a recurrent rash on his lower legs. He has a past medical history of a brief period of intravenous drug use in his early twenties. He currently takes no medications. Vital signs are within normal limits. Physical examination reveals a non-blanching, palpable purpuric rash over the bilateral shins and ankles. There is mild symmetric tenderness and swelling of the metacarpophalangeal and proximal interphalangeal joints.

Laboratory evaluation is as follows:

TestResultReference Range
Hemoglobin12.4 g/dL12.4\text{ g/dL}13.5−17.5 g/dL13.5-17.5\text{ g/dL}
Platelets185,000/mm3185,000/\text{mm}^3150,000−400,000/mm3150,000-400,000/\text{mm}^3
Creatinine1.7 mg/dL1.7\text{ mg/dL}0.6−1.2 mg/dL0.6-1.2\text{ mg/dL}
AST58 U/L58\text{ U/L}10−40 U/L10-40\text{ U/L}
ALT72 U/L72\text{ U/L}10−40 U/L10-40\text{ U/L}
Rheumatoid factorPositiveNegative
Complement C3105 mg/dL105\text{ mg/dL}80−160 mg/dL80-160\text{ mg/dL}
Complement C46 mg/dL6\text{ mg/dL}15−45 mg/dL15-45\text{ mg/dL}

Urinalysis shows 2+ protein, 15−2015-20 red blood cells/hpf, and occasional red blood cell casts.

Which of the following is the most appropriate next step in the diagnostic evaluation of this patient?

  1. A
    Antineutrophil cytoplasmic antibody assay
  2. Testing for hepatitis C virus RNAAnswer
  3. C
    Antinuclear antibody assay
  4. D
    Testing for hepatitis B surface antigen
  5. E
    Blood cultures and transesophageal echocardiography

Answer

Testing for hepatitis C virus RNA
The correct answer is testing for hepatitis C virus RNA. This patient exhibits the classic features of mixed cryoglobulinemia syndrome (MCS): palpable purpura, arthralgias, and immune-complex mediated glomerulonephritis (nephritic syndrome). The laboratory findings of a positive rheumatoid factor (an IgM antibody against IgG) and an isolated low C4 with a normal C3 are the pathophysiological hallmarks of this condition. More than 90% of MCS cases are secondary to chronic hepatitis C virus infection, making HCV screening the most essential next step in diagnostic evaluation.

Step-by-Step Solution

1
Analyze the clinical presentation and physical examination findings.
The patient has the classic triad of palpable purpura, weakness (fatigue), and arthralgias, along with nephritic syndrome (RBC casts, elevated creatinine, proteinuria).
Identifying this triad points strongly toward a systemic small-to-medium vessel vasculitis or immune-complex mediated disease.
2
Interpret the specific immunological laboratory profile.
The labs show a positive rheumatoid factor (RF), a normal C3 level, and a severely depressed C4 level.
In this context, the RF is actually an IgM autoantibody directed against the Fc portion of IgG. These form immune complexes (cryoglobulins) that robustly activate the classical complement pathway, leading to marked consumption of C4 but often sparing C3.
3
Determine the underlying etiology of the suspected diagnosis (Mixed Cryoglobulinemia Syndrome).
Mixed cryoglobulinemia is overwhelmingly associated with chronic Hepatitis C virus (HCV) infection.
Chronic immune stimulation by HCV drives the clonal expansion of B cells that produce cryoglobulins. Testing for HCV RNA is the most critical next diagnostic step to confirm the etiology before initiating antiviral and/or immunosuppressive therapy.

Key Concept

Mixed cryoglobulinemia syndrome presents with palpable purpura, arthralgias, and glomerulonephritis. It is characterized immunologically by a positive rheumatoid factor and an isolated low C4, and requires testing for its most common underlying cause: chronic hepatitis C virus infection.
Estimated Time:1m 30s
Rate this question

Topics

Rheumatology and Immunology

More questions from this topic