Given the task definition and input, reply with output. Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.

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.

Which deficiency is the cause of restless leg syndrome?