The 'String of beads' or sausage appearance of the renal aery is due to :

String of beads or sausage appearance is seen in medial fibroplasia. It is produced by the stenotic segments where media is replaced by collagen with intervening aneurysms at the site of medial weakening. Medial fibroplasia with aneurysms is the most frequent and distinctive form of fibromuscular dysplasia. It typically involves the distal two thirds of the renal aery and may also affect segmental aeries within the kidney. Paramedian fibroplasia is the second most common form of fibromuscular dysplasia. It is characterized by an irregular pattern of fibrosis that replaces the outer one half to two thirds of the media by fibrous tissue. Ref: Diagnostic Radiology : Advances in Imaging By Manorama Berry, page 251, Urologic Surgical Pathology By David G. Bostwick, page 37.