31. A healthy 28-year-old, P0+2, has a 7-hour history of sudden-onset severe right-sided lower abdominal pain; her LMP was 6 weeks ago; periods are normally infrequent and irregular: 3–4/35–56 days. She and her partner have a past history of subfertility and are currently receiving clomiphene citrate 100mg for ovulation induction for anovulatory oligomenorrhoea. Her first pregnancy was an early miscarriage; the second was a left tubal pregnancy.
On examination: she looks pale and distressed; the abdomen is very tender in the lower half on the right side with rebound peritonism; no masses. Vaginal examination: normal sized uterus, no masses and cervical excitation tenderness is elicited.
Observations are as follows:
Pulse rate (bpm) 110
Temperature (ºC) 36.0
Blood pressure (mmHg) 120/50
Urine analysis has not passed urine
Respiratory rate (breaths/min) 14
O2 saturation on air (%) 96
What is the most appropriate investigation. Of choice is