Q: Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
The restless legs syndrome is characterized by an unpleasant sensation in the legs which causes an imperative need to move the legs and is therefore considered to be a disorder of movement. When it appears before going to sleep, it may interfere with falling asleep and lead to a sleep-deficit. It is a clinical condition with a satisfactory treatment, and improvement of the associated sleep disorder. The etiology is unknown, sometimes it is familial. The syndrome is increasingly often diagnosed, particularly in association with iron deficiency, during pregnancy, in chronic renal failure and in patients with peripheral neuropathy. Polysomnography is not necessary, unless one suspects an associated disorder of periodic leg movements. Treatment is by dopaminergic, opiate, benzodiazepine, anticonvulsant drugs or clonidine.
A:
Which deficiency is the cause of restless leg syndrome?