Diagnosis of Primary Amenorrhea and Pelvic Mass

The patient presents with classic signs suggesting an obstruction in the female reproductive tract's outflow. Key features include:

• Age: 16 years
• Primary Amenorrhoea: No menstrual periods ever started.
• Periodic Pain: Cyclical pain occurring at expected intervals.
• Suprapubic Mass: A palpable mass extending towards the umbilicus.

Evaluating the Options

Cryptomenorrhoea

This condition refers to menstruation that is hidden or cannot exit the body. Common causes include an imperforate hymen or a transverse vaginal septum.

• Primary Amenorrhoea: The blockage prevents menstrual blood from exiting.
• Periodic Pain: Monthly accumulation of blood behind the obstruction (hematocolpos or hematometra) causes cyclic pain.
• Suprapubic Mass: The accumulated blood forms a distensible mass, often palpable abdominally.

This diagnosis aligns perfectly with all the patient's symptoms and examination findings.

Large Ovarian Cyst

While a large ovarian cyst can cause a palpable mass and pain, it typically does not cause primary amenorrhoea unless it's hormonally active or causing significant compression leading to secondary amenorrhoea. It doesn't inherently explain the periodic nature related to menstruation.

Bladder-Neck Hypertrophy

This condition affects the bladder outlet and is not associated with amenorrhoea or the described symptoms in a young female.

Uterine Leiomyoma

Uterine fibroids can cause a mass and pain, and sometimes abnormal bleeding, but primary amenorrhoea is an atypical presentation. They usually occur later in reproductive life.

Conclusion

Given the combination of primary amenorrhoea, periodic pain, and a suprapubic mass in a 16-year-old girl, cryptomenorrhoea due to an outflow tract obstruction is the most likely diagnosis.