A 26-year-old woman presented with subacute atraumatic pain and swelling of her right knee and left foot. The right knee was erythematous and warm with an associated joint effusion; the second and third metatarsophalangeal joints and extensor tendons of the left foot were erythematous, warm, and tender. There were well-demarcated erythematous plaques involving the posterior neck. The nail bed of the right fifth digit had a circular area of yellow discoloration with a well-demarcated rim of brown discoloration, resembling a drop of oil (see Fig. 1). Synovial fluid from the knee contained 15,541 WBC/μL, and no organisms were identified. Serologic workup was negative, including antinuclear antibody, anti-double stranded DNA, and rheumatoid factor. Urine and synovial fluid nucleic acid amplification tests for Neisseria gonorrhoeae were negative.

Figure 1
figure 1

A circular area of discoloration involving the nail bed of the right fifth digit, known as the oil drop sign (arrows, images 1 and 2).

Full size image

The oil drop sign is caused by nail bed parakeratosis and psoriasiform hyperplasia and is a physical finding of psoriasis (1,2,3). Nail psoriasis is common in patients with psoriatic arthritis and can involve the nail matrix (e.g., pitting, trachyonychia, dystrophy, leukonychia) or the nail bed (e.g., onycholysis, subungual hyperkeratosis, oil drop, splinter hemorrhages) (4, 5). This patient was diagnosed with psoriasis and started on methotrexate, etanercept, and diclofenac, resulting in a steady improvement in symptoms.