In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Two patients with similar clinical features are presented : both patients had chronic renal failure , on hemodialysis for many years but recently begun on a high - flux dialyzer ; both had been receiving a carbidopa / levodopa preparation ; and both had the onset of hallucinosis and recurrent seizures , which were refractory to anticonvulsants .
### Output Text:
Two patients with similar clinical features are presented : both patients had chronic renal failure , on hemodialysis for many years but recently begun on a high - flux dialyzer ; both had been receiving a <span class="chemical">carbidopa / levodopa</span> preparation ; and both had the onset of hallucinosis and recurrent seizures , which were refractory to anticonvulsants . 