Pelvic Inflammatory Disease (PID): Given the patient's symptoms of pain during urination, vaginal itching, irritation, and green/yellow odorous vaginal discharge, PID is a strong consideration. The recent history of a possible spontaneous abortion and the presence of mild abdominal cramping, bleeding, and tissue in the discharge further support this diagnosis, as PID can be a complication of an incomplete abortion.

Urinary Tract Infection (UTI): The patient's complaint of pain during urination (dysuria) is a common symptom of UTIs. Although the presence of vaginal discharge and itching might suggest a different primary issue, UTIs can coexist with or exacerbate other conditions.
Vaginitis: This could be due to bacterial vaginosis (BV), vulvovaginal candidiasis, or trichomoniasis, given the symptoms of vaginal itching, irritation, and abnormal discharge. The green/yellow odor is particularly suggestive of BV or trichomoniasis.
Incomplete Abortion: The patient's history of a recent possible spontaneous abortion with ongoing bleeding, cramping, and tissue in the discharge suggests that she may have an incomplete abortion, which requires immediate medical attention.

Ectopic Pregnancy: Although the patient reports a recent possible spontaneous abortion, the possibility of an ectopic pregnancy must be considered, especially given her recent pregnancy and symptoms of abdominal cramping and bleeding. Ectopic pregnancy is a medical emergency.
Septic Abortion: If the patient did indeed have an incomplete or spontaneous abortion, there is a risk of septic abortion, especially if the abortion was incomplete or if she underwent an unsafe abortion procedure. This condition is life-threatening and requires immediate intervention.
Sexually Transmitted Infections (STIs): Given the patient's age and symptoms of vaginal discharge and itching, STIs such as gonorrhea or chlamydia must be considered. These infections can lead to serious complications if not treated promptly.

Cervical or Endometrial Cancer: Although extremely rare in a 16-year-old, any abnormal vaginal bleeding, especially in the context of a recent pregnancy, should prompt consideration of malignancy, albeit very low on the differential list.
Tuberculous Endometritis: This is a rare condition that could present with abnormal vaginal discharge, bleeding, and systemic symptoms, but it would be an unusual diagnosis in this demographic without other suggestive history or exposure.