Post-viral syndrome: Given the recent history of viral infection symptoms (sore throat, persistent headache, dry cough, occasional febrile feeling) that could have been COVID or a cold, and the onset of pain and weakness after this illness, a post-viral syndrome is a plausible explanation. The rheumatologist's suspicion of this condition further supports its consideration as the most likely diagnosis.

Fibromyalgia: Characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, memory, and mood issues. The pain's persistence, lack of response to NSAIDs and prednisone, and the absence of stiffness or improvement with movement could align with fibromyalgia, especially given the diffuse nature of the pain across the back and shoulders.
Myofascial Pain Syndrome: This condition involves pain and inflammation in the fascia (the connective tissue surrounding muscles). The severe pain in the shoulders and upper back, along with arm weakness, could be indicative of myofascial pain syndrome, particularly if trigger points are identified.
Neuropathic Pain: The arm weakness and the fact that the pain is not affected by movement or palpation might suggest a neuropathic component to her pain. Conditions affecting the nerves can cause both pain and weakness without necessarily being accompanied by other signs of inflammation or structural damage.

Multiple Myeloma or Other Malignancies: Although the physicians do not think it is metastasis in the spine due to the symmetrical effects and diffuse pain, it is crucial not to miss a diagnosis of multiple myeloma or other malignancies. These conditions can present with bone pain and, less commonly, with neurological symptoms due to nerve compression or infiltration.
Spinal Cord Compression: This is a medical emergency that requires immediate attention. Symptoms can include pain, weakness, and sensory changes in the arms or legs. Given Tuula's arm weakness, even though the lower body is unaffected, spinal cord compression needs to be considered, especially with the upcoming MRI.
Infectious Diseases (e.g., Osteomyelitis, Discitis): Despite the normal CRP and ESR, infectious processes can sometimes present with minimal laboratory abnormalities early in the course. The history of a tick bite and consideration of Lyme disease, although seemingly ruled out, highlights the importance of considering infectious etiologies.

Ehlers-Danlos Syndrome (EDS) or Other Connective Tissue Disorders: These conditions can present with widespread pain, joint hypermobility, and other systemic symptoms. Although less likely, given the acute onset and specific pattern of Tuula's symptoms, EDS or other connective tissue disorders could be considered if other diagnoses are ruled out.
Sarcoidosis: This condition can cause musculoskeletal pain, among other symptoms, due to granulomatous inflammation. It is less likely given the normal blood tests and lack of other systemic symptoms but remains a rare possibility to consider if the diagnosis remains elusive.