Final answer:
The additional feature likely present is fever and jaundice, which suggests possible liver involvement such as a liver abscess or metastasis, especially given the patient's history of breast cancer and recent chemotherapy.
Step-by-step explanation:
The additional feature that might be present in the 53-year-old woman with right-sided abdominal pain and a medical history of breast cancer is fever and jaundice. Considering the severe, constant, and burning nature of the pain that interferes with sleep and the completed chemotherapy, it suggests a possible liver involvement, which could be indicative of a liver abscess or metastasis.
A liver span of 8cm and hypersensitivity to light brushing (indicative of peritoneal irritation) further support the suspicion of liver involvement. Signs such as fever and jaundice may accompany liver pathology as seen in conditions such as cholangitis, pancreatitis, or metastatic cancer. The clinical history presented does not strongly suggest hollow viscus perforation, ascites, black stool indicative of an upper GI bleed, or skin lesions typically associated with skin conditions or infections.