MODEL RELEASED. Granuloma from a pilonidal sinus. Close-up of a granuloma (nodule just below centre) that has developed from the partial healing of a pilonidal sinus. A granuloma is a mass of tissue that can form at the site of an infection. A pilonidal sinus is an infected pit, typically in the cleft between the buttocks, that forms when a hair penetrates the skin and allows bacteria to enter and multiply. They are more common in hairy people and those who sit down for long periods. Treatment is with surgical excision of the entire infected area.