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.

Example input: Context: Architecturally, the school has a Catholic character. Atop the Main Building's gold dome is a golden statue of the Virgin Mary. Immediately in front of the Main Building and facing it, is a copper statue of Christ with arms upraised with the legend 'Venite Ad Me Omnes'. Next to the Main Building is the Basilica of the Sacred Heart. Immediately behind the basilica is the Grotto, a Marian place of prayer and reflection. It is a replica of the grotto at Lourdes, France where the Virgin Mary reputedly appeared to Saint Bernadette Soubirous in 1858. At the end of the main drive (and in a direct line that connects through 3 statues and the Gold Dome), is a simple, modern stone statue of Mary.
Question: To whom did the Virgin Mary allegedly appear in 1858 in Lourdes France?
Example output: Saint Bernadette Soubirous
Example explanation: This is the correct answer. Because the question is asking for the name of the person and following sentence from the passage answers it: It is a replica of the grotto at Lourdes, France where the Virgin Mary reputedly appeared to Saint Bernadette Soubirous in 1858.
Q: 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?
A:
iron