What is the most likely differential diagnosis for a patient presenting with bilateral hand pain affecting the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, reduced motor and fine hand function, a history of eye dryness, and joint distortion on X-rays?

Most Likely Differential Diagnosis: Psoriatic Arthritis

The most likely diagnosis is C. Psoriatic arthritis, given the combination of bilateral DIP and PIP joint involvement with joint distortion on X-rays, reduced hand function, and eye dryness (which may represent associated uveitis or sicca symptoms). 1

Key Diagnostic Reasoning

Joint Distribution Pattern is Critical

• Psoriatic arthritis characteristically targets DIP joints, which is uncommon in rheumatoid arthritis but typical for psoriatic arthritis 1
• The combination of both DIP and PIP involvement with joint distortion strongly suggests psoriatic arthritis over other inflammatory arthropathies 1
• Rheumatoid arthritis mainly targets MCPs, PIPs, and wrists—DIP involvement is distinctly uncommon 1

Why Not the Other Options

Rheumatoid Arthritis (Option B) is less likely because:

• RA rarely involves DIP joints; it predominantly affects MCPs and PIPs with relative DIP sparing 1
• While RA can cause joint distortion, the DIP involvement pattern argues strongly against this diagnosis 1
• The bilateral symmetric pattern could fit RA, but the DIP involvement is the critical distinguishing feature 1

Sjögren syndrome (Option A) is unlikely because:

• Sjögren's is primarily a sicca syndrome (dry eyes, dry mouth) that may have associated arthralgia, but typically does not cause destructive joint disease with radiographic distortion 1
• While eye dryness is present, the prominent destructive arthropathy with joint distortion is not characteristic of primary Sjögren's 1

Systemic lupus erythematosus (Option D) is unlikely because:

• SLE typically causes non-erosive arthritis without the joint distortion described 1
• The pattern of DIP and PIP involvement with radiographic changes is not characteristic of lupus arthritis 1

Clinical Pearls for Psoriatic Arthritis

• Dactylitis (sausage digits) and enthesitis are additional features that support psoriatic arthritis, though not mentioned in this case 1
• Psoriatic arthritis can affect just one ray or multiple joints asymmetrically, though bilateral involvement occurs 1
• DIP involvement in psoriatic arthritis represents true joint inflammation, not the bony spurs (Heberden's nodes) seen in osteoarthritis 1
• Look for psoriatic skin plaques or nail changes (pitting, onycholysis) to confirm the diagnosis 1

Important Caveat: Erosive Hand Osteoarthritis

Erosive osteoarthritis must be considered as an alternative diagnosis:

• Erosive OA targets IPJs (both DIP and PIP) and shows radiographic subchondral erosion with joint distortion 1
• It typically has abrupt onset, marked pain, inflammatory symptoms (stiffness, soft tissue swelling), and worse functional impairment than non-erosive OA 1
• However, erosive OA typically occurs in patients over 40 and lacks the systemic features (eye symptoms) 1
• The eye dryness in this case tips the balance toward an inflammatory arthropathy with systemic features rather than erosive OA 1

Next Diagnostic Steps

• Examine for psoriatic skin lesions and nail changes carefully 1
• Assess for dactylitis and enthesitis (Achilles tendon, plantar fascia insertion points) 1
• Consider inflammatory markers (ESR, CRP), though these may be only mildly elevated 1
• Rheumatoid factor and anti-CCP antibodies should be negative in psoriatic arthritis 1
• Ophthalmology referral to characterize the eye symptoms (uveitis vs sicca) 1