Final answer:
The patient's symptoms are consistent with appendicitis, and the most likely subsequent complication is peritonitis.
Step-by-step explanation:
The symptoms described in the question are consistent with appendicitis. Appendicitis is characterized by the inflammation of the appendix, a small pouch connected to the large intestine. The lower right abdominal pain that started gradually and became suddenly unbearable, along with the patient's other symptoms such as the fetal position, elevated heart rate, and low blood pressure, indicate a possible diagnosis of appendicitis.
Out of the given options of subsequent complications, the most likely one is B. peritonitis. Peritonitis is a life-threatening condition that results from inflammation or infection of the peritoneum, the lining of the abdominal cavity. In severe cases of appendicitis, the appendix can rupture, causing the release of bacteria and fecal matter into the abdominal cavity, leading to peritonitis.