You will be given a definition of a task first, then some input of the task.
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.

Context: Restless leg syndrome (RLS) and periodic limb movement disorder (PLMD) are considered to be a continuum of a neurological sleep disorder associated with abnormal iron metabolism or deficiency. I describe a case of RLS and PLMD in a cystic fibrosis patient with iron deficiency from chronic hemoptysis. This is the first case that reports RLS and PLMD manifesting from iron deficiency caused by chronic hemoptysis in advanced cystic fibrosis lung disease.
Question: Which deficiency is the cause of restless leg syndrome?
Output:
iron