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: Our aim was to observe if patients with panic disorder ( PD ) and patients with major depression with panic attacks ( MDP ) ( Diagnostic and Statistical Manual of Mental Disorders , Fourth Edition criteria ) respond in a similar way to the induction of panic attacks by an oral caffeine challenge test .
### Output Text:
Our aim was to observe if patients with panic disorder ( PD ) and patients with major depression with panic attacks ( MDP ) ( Diagnostic and Statistical Manual of Mental Disorders , Fourth Edition criteria ) respond in a similar way to the induction of panic attacks by an oral <span class="chemical">caffeine</span> challenge test . 