This patient is pregnant in the first trimester presenting to the Emergency department with right lower quadrant pain. Any first trimester pregnant patient with abdominal pain should be evaluated for ectopic pregnancy. Other causes of this symptom include ovarian torsion, ovarian cyst rupture, pelvic inflammatory disease, tubo-ovarian abscess, urinary tract infection, ureterolithiasis, colitis, or appendicitis. An intra-uterine pregnancy is confirmed on transvaginal ultrasound which excludes ectopic pregnancy from the differential. Ovarian pathologies are also investigated on the ultrasound and are not discovered. 

Another common diagnosis based on the patient’s pain location, young age, and markedly tender abdomen is acute appendicitis. The most common presenting symptom in appendicitis is right lower quadrant pain. Other signs include fever, anorexia, nausea, or vomiting.  Pregnant women may present with back or flank pain, rather than right lower quadrant pain, as the uterus may displace the appendix in the abdomen. There is no single symptom or laboratory test that can reliably exclude the diagnosis of appendicitis. The gold standard test for acute appendicitis diagnosis is a CT scan of the abdomen with IV contrast dye. PO or PR contrast are additionally used in some institutions based on preference and protocols.  In children, appendiceal ultrasound is performed first to avoid excessive radiation exposure and financial cost. CT scanning (Choice A) is similarly avoided in first-trimester pregnancy to diagnose appendicitis, although it is the test of choice in non-pregnant adults. MRI imaging of the abdomen and pelvis (Choice C) is another diagnostic option for pregnant patients, but this is not recommended until an ultrasound is performed. IV antibiotics (Choice D) may be needed to treat appendicitis or other abdominal infections, but this patient lacks a definitive diagnosis or signs of sepsis or shock which would support emergent antibiotics. The best next step to further evaluate the cause of this patient’s symptoms is conducting an appendiceal ultrasound (Choice B). If this study is non-conclusive or is not available, an MRI should be performed. 

Emergency department treatment for acute appendicitis is IV antibiotics, IV hydration, and surgical consultation for appendectomy. Immediate surgery may be avoided in patients who present several days after symptom onset or with a ruptured appendix. These cases are treated with IV antibiotics, IV hydration, bowel rest, and close monitoring.