A 20-year-old female with a known history of sickle cell disease presents to the general medical ward with a 3-day history of escalating pain in her right thigh and lower back. She reports a low-grade fever, chills, and increasing difficulty bearing weight. She has experienced similar painful crises previously, but this pain feels distinct and more localized. Her last hospitalization was 6 months ago for a vaso-occlusive crisis. On examination, her vital signs are: blood pressure 128/80 mmHg, pulse 102/min, respiratory rate 18/min, and temperature 38.9°C. Her right femur is exquisitely tender to palpation, and she winces with any attempt at passive range of motion. There is no overlying erythema or swelling. Her low back has diffuse tenderness but no focal neurological deficits are noted. Initial imaging reveals periosteal reaction and bone marrow edema consistent with osteomyelitis of the right femur. What is the most appropriate initial empiric antibiotic regimen and what is the most critical monitoring parameter for therapy response?