( ( Some ( of ( the ( ( most common ) ( bacterial STIs ) ) ) ) ) ( ( are ( chlamydia ( , ( gonorrhea ( , ( and syphilis ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT Some)) (PP (IN of) (NP (DT the) (ADJP (RBS most) (JJ common)) (JJ bacterial) (NNS STIs)))) (VP (VBP are) (NP (NN chlamydia) (, ,) (NN gonorrhea) (, ,) (CC and) (NN syphilis))) (. .)))
Some of the most common bacterial STIs are chlamydia, gonorrhea,  and syphilis.
(ROOT (S (NP (NNP Chlamydia) (, ,) (NNP gonorrhea) (, ,) (CC and) (NNP syphilis)) (VP (VBP are) (NP (NP (NNS examples)) (PP (IN of) (NP (JJ bacterial) (NNS stis))))) (. .)))