Final answer:
The organism that can cause painful ulcerative genital lesions in the described case is Herpes simplex (D). Other options provided, like Chlamydia trachomatis and Treponema pallidum, either cause asymptomatic infections or non-ulcerative lesions or painless chancres, respectively.
Step-by-step explanation:
The organism most related to the symptoms described by the 23-year-old female patient, particularly painful genital lesions, fever, chills, myalgia, and signs of meningitis (headache, photophobia, stiff neck), in conjunction with negative Gram stain and elevated white cell count in the CSF, is Herpes simplex (D). Herpes simplex virus (HSV) causes painful vesicular and ulcerative lesions in the genital area. Chlamydia trachomatis (D-K), while a common STI, typically presents with asymptomatic infections or non-ulcerative cervicitis in women. Treponema pallidum, which causes syphilis, is also a cause of genital lesions, but these are usually painless chancres in primary syphilis, which is not consistent with the painful lesions described in this scenario. Papillomavirus causes warts, not ulcerative lesions, and Group B strep is more commonly associated with neonatal infections and not commonly responsible for genital ulcers.