A 58-year-old female patient with shoulder and pelvic girdle pain and recent weight loss is most likely diagnosed with?

The presentation of shoulder and pelvic girdle pain alongside recent weight loss is indicative of polymyalgia rheumatica. This condition typically affects older adults and is characterized by bilateral musculoskeletal pain and stiffness, particularly in the shoulder and hip areas. The weight loss observed in this patient is also consistent with polymyalgia rheumatica, as systemic inflammation often accompanies the condition, which may lead to unintentional weight loss.

Polymyositis, on the other hand, primarily affects muscle strength and is associated more with symmetrical muscle weakness rather than generalized pain, along with signs of muscle inflammation, which would not be as prominent in this patient's case. Osteoarthritis usually presents with joint pain and stiffness, but weight loss is not a hallmark feature, and the age and nature of the complaints make this option less likely. Fibromyalgia is characterized by widespread musculoskeletal pain and other symptoms such as fatigue and sleep disturbances, but again, it does not typically present with the significant weight loss seen in this scenario.

Thus, the symptoms and clinical findings align most closely with polymyalgia rheumatica, making it the most likely diagnosis for this patient.