Which condition is characterized by tissue growing inside the uterus?

The condition characterized by tissue growing inside the uterus is endometriosis. In this disorder, tissue similar to the lining of the uterus (endometrium) grows outside the uterus, often leading to pain, irregular bleeding, and can contribute to infertility. While the other options deal with reproductive health, they do not describe the presence of tissue specifically growing within the uterus.

Pelvic inflammatory disease (PID) refers to an infection of the female reproductive organs, which is not related to the growth of tissue but rather to inflammatory and infectious processes. Fibroids are benign tumors that grow in or on the uterus, but they are not the same as endometrium-like tissue growing inappropriately outside the uterine lining. Polycystic ovary syndrome (PCOS) is a hormonal disorder affecting ovulation and is also distinct from the scenario described in the question.

Thus, the hallmark of endometriosis is that it involves endometrial-like tissue existing in places where it normally shouldn't, aligning perfectly with the description of abnormal tissue growth as presented in the question.