Dermatology MCQ - Inflammatory Dermatoses - PSORIATIC ARTHRITIS

A 55-year-old man with a long history of plaque psoriasis presents with pain and stiffness in his hands and lower back. On examination, you note "sausage-like" swelling of two fingers on his right hand and pitting of several fingernails. Radiographs of his hands show "pencil-in-cup" deformities and joint space narrowing. Which of the following is the most likely diagnosis and the strongest genetic association for this condition?

A) Psoriatic Arthritis; HLA-B27
B) Rheumatoid Arthritis; HLA-DR4
C) Osteoarthritis; no significant HLA association
D) Reactive Arthritis; HLA-B27
E) Gout; no significant HLA association

Correct Answer: A) Psoriatic Arthritis; HLA-B27

Explanation

This presentation is classic for Psoriatic Arthritis (PsA), an inflammatory arthritis associated with psoriasis.

Key Clinical Features of Psoriatic Arthritis:

• Musculoskeletal Patterns: PsA is heterogeneous, but this case exhibits classic findings:

  • Dactylitis: "Sausage-like" swelling of an entire digit, due to inflammation of both joints and tendon sheaths.

  • Spondylitis: Inflammation of the spine (causing lower back pain and stiffness).

  • Distal Interphalangeal (DIP) Joint Involvement: A characteristic feature, often associated with nail changes.

• Nail Findings: Nail pitting is a highly specific finding linking the arthritis to psoriasis. Onycholysis, hyperkeratosis, and oil-drop spots are also common.

• Radiographic Findings: "Pencil-in-cup" deformity, where the proximal bone is whittled down to a sharp point (pencil) that fits into a concave erosion of the adjacent bone (cup), is a pathognomonic feature of PsA. Joint space narrowing and bony proliferation are also seen.

Genetic Association:

• The strongest genetic association for psoriatic arthritis, particularly the spondylitis and DIP joint patterns, is with HLA-B27.

• HLA-C*06:02 is the primary genetic risk factor for cutaneous psoriasis itself, but HLA-B27 is a stronger driver for the development of the inflammatory arthritis, especially axial disease.

Why Not the Other Options?

• (B) Rheumatoid Arthritis (RA): Typically presents with a symmetrical, polyarticular arthritis affecting small joints of the hands and feet. Dactylitis and DIP involvement are rare. It is associated with HLA-DR4. Radiographs show periarticular osteopenia and erosions without the "pencil-in-cup" deformity or bony proliferation.

• (C) Osteoarthritis: A degenerative, non-inflammatory arthritis. It causes Heberden's and Bouchard's nodes (bony enlargement) and joint space narrowing, but not dactylitis, nail pitting, or "pencil-in-cup" deformities.

• (D) Reactive Arthritis: Can cause dactylitis, spondylitis, and is strongly associated with HLA-B27. However, it is typically triggered by an infection (genitourinary or gastrointestinal) and lacks the characteristic psoriatic plaques and nail pitting. The presence of established psoriasis makes PsA the correct diagnosis.

• (E) Gout: Causes an acute, intensely painful monoarthritis (often the first MTP joint). It is caused by urate crystal deposition, not associated with HLA, and does not cause dactylitis or the specific radiographic changes seen here.

Management:

• NSAIDs for pain and inflammation.

• Disease-modifying antirheumatic drugs (DMARDs) like methotrexate.

• Biologics (e.g., TNF-alpha inhibitors, IL-17 inhibitors) are highly effective.

Note: The combination of psoriasis, dactylitis, nail pitting, and characteristic radiographic findings is diagnostic for PsA. While HLA-C*06:02 is key for skin disease, HLA-B27 is the critical genetic link for the development of the arthropathy, especially when spondylitis is present.