Green discoloration of a dishwasher's fingernail

An otherwise healthy 60-year-old dishwasher presents with asymptomatic green discoloration of his right middle fingernail over a period of 2 months. The discoloration began from the distal margin. He denies wearing gloves while working and reports a tendency to pick at his nails when nervous. On examination, distal onycholysis and green discoloration are present exclusively on the right middle nail ...