Instructions: Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.
Input: Context: Restless legs syndrome (RLS) is a clinically important, common disease and should be diagnosed and treated early and adequately. At present, there have been no clinical biomarkers or methodologies that can contribute to the correct diagnosis of RLS, RLS should be diagnosed on the basis of 4 essential criteria: urge to move the legs, improvement after movement, and worsening or occurrence of symptoms in the evening and at rest. When applying the criteria, RLS mimics should be ruled out and comorbid diseases should be taken into account. The origin and pathogenesis of RLS are still under investigation; however, iron deficiency in the brain has been observed on imaging and cerebrospinal fluid analyses of patients with RLS. In contrast, the results of neuroimaging studies evaluating brain dopaminergic functions in patients with RLS have yielded inconclusive results, although involvement of the hypothalamus (A11) is thought to cause impaired dopaminergic modulation in the dorsal horn and intermediolateral nucleus, resulting in the restlessness of legs.
Question: Which deficiency is the cause of restless leg syndrome?
Output:
iron