A 30-year-old nulliparous female presented with menorrhagia, and an ultrasound scan shows a centrally lying pelvic mass. What is the most likely diagnosis? Provide differential diagnoses.

The most likely diagnosis in this case is a uterine fibroid (leiomyoma).

Differential diagnoses for a centrally lying pelvic mass in a 30-year-old nulliparous female with menorrhagia may include:
1. Adenomyosis: a condition where the endometrial tissue grows into the muscular wall of the uterus, causing heavy menstrual bleeding and pelvic pain.
2. Endometrial polyp: a growth of tissue in the lining of the uterus that can cause abnormal uterine bleeding.
3. Endometrial hyperplasia: an overgrowth of the endometrium, which can cause heavy menstrual bleeding and is a risk factor for endometrial cancer.
4. Ovarian cyst: a fluid-filled sac on the ovary that can cause pelvic pain and abnormal bleeding.
5. Endometrial cancer: although less common in younger women, it is important to consider as a possibility in cases of abnormal uterine bleeding.

Most Likely Diagnosis:

- Uterine fibroid

Differential Diagnoses:

- Adenomyosis: A condition where endometrial tissue invades the muscular wall of the uterus.
- Endometrial hyperplasia: An abnormal thickening of the uterine lining.
- Endometrial polyp: A benign growth that protrudes from the inner lining of the uterus.
- Cervical cancer: A malignant tumor originating from the cervix.
- Ovarian cancer: A malignant tumor arising from the ovaries.
- Leiomyoma: A benign tumor of smooth muscle origin, which can occur in the uterus or other reproductive organs.
- Adenocarcinoma of the uterus: A malignant tumor of the endometrial lining.