Vulvovaginal candidiasis: This is a common cause of vaginitis, especially in young women. Although the workup is negative, it's possible that the diagnosis was missed or the patient may not have been symptomatic at the time of testing.

Bacterial vaginosis: Another common cause of vaginitis, which may not always be detected by standard workups.
Trichomoniasis: A sexually transmitted infection that can cause vaginitis, which may have been missed if the workup did not include specific testing for this condition.
Atrophic vaginitis: A condition caused by decreased estrogen levels, which can lead to vaginitis symptoms, especially in women who are breastfeeding or postmenopausal, but can also occur in younger women.

Cervical cancer: Although rare in young women, it's essential to rule out cervical cancer, especially if the patient has a history of abnormal Pap smears or has not been screened regularly.
Pelvic inflammatory disease (PID): A serious condition that can cause vaginitis symptoms, PID requires prompt treatment to prevent long-term complications.
Sexually transmitted infections (STIs) such as gonorrhea or chlamydia: These infections can cause vaginitis symptoms and have serious consequences if left untreated.

Desquamative inflammatory vaginitis: A rare condition characterized by intense inflammation and desquamation of the vaginal epithelium.
Lichen planus: A chronic inflammatory condition that can affect the vagina and cause vaginitis symptoms.
Lichen sclerosus: A rare condition that can cause thinning and scarring of the vaginal tissue, leading to vaginitis symptoms.