Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Example: In 1582 the united provinces invited francis, duke of anjou to lead them; but after a failed attempt to take antwerp in 1583, the duke left the netherlands again. after the assassination of william of orange (10 july 1584), both henry iii of france and elizabeth i of england declined the offer of sovereignty. however, the latter agreed to turn the united provinces into a protectorate of england (treaty of nonsuch, 1585), and sent the earl of leicester as governor-general. this was unsuccessful and in 1588 the provinces became a confederacy. the union of utrecht is regarded as the foundation of the republic of the seven united provinces, which was not recognized by the spanish empire until the peace of westphalia in 1648.
Example solution: In what year did francis, duke of anjou leave the netherlands?
Example explanation: The generated question is perfect. Because the question is based on second sentence of the input passage and its answer is 1583.

Problem: To investigate the clinical feature of Parkinson's disease (PD) with restless leg syndrome (RLS) and the pathogenesis of RLS. We conducted a cross-sectional and control study. The case group concluded 31 PD with RLS patients, meanwhile 39 PD patients were selected as the control group. Clinical history, clinical manifestations, complications and laboratory examinations were compared respectively between the two groups. All the RLS symptoms did not appear in RLS patients until the PD symptoms came out. Significant differences were found in complications such as swallow disturbance, constipation and illusion, when we compared the two PD groups (P < 0.05). Compared with the PD or healthy group, the level of serum ferritin and the H-reflex latency of tibial nerve were significantly decreased in PD with RLS group (P < 0.05). Secondary RLS is a complication of PD. Deficiency of iron and decreased inhibition function of spinal cord may lead to the occurrence of RLS in PD patients. When their motor symptoms are serious and complications are more common, PD patients are more possible to have RLS symptoms.

Solution: Which deficiency is the cause of restless leg syndrome?