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: A total of 58 . 6 % ( n = 17 ) of patients with PD , 44 . 4 % ( n = 12 ) of patients with MDP , 12 . 0 % ( n = 3 ) of patients with MD , and 7 . 1 % ( n = 2 ) of control subjects had a panic attack after the 480 - mg caffeine challenge test ( chi ( 2 ) ( 3 ) = 16 . 22 , P = . 001 ) .
### Output Text:
A total of 58 . 6 % ( n = 17 ) of patients with PD , 44 . 4 % ( n = 12 ) of patients with MDP , 12 . 0 % ( n = 3 ) of patients with MD , and 7 . 1 % ( n = 2 ) of control subjects had a panic attack after the 480 - mg <span class="chemical">caffeine</span> challenge test ( chi ( 2 ) ( 3 ) = 16 . 22 , P = . 001 ) . 