A 70-year-old female patient presents with a 3-month history of headache and 1-month history of visual acuity decline. She also reports low-grade fever, fatigue, and generalized flu-like symptoms. No history of hypertension (HTN). Physical exam reveals nodular skin in the temporal region, marked tenderness over the left temporal artery, and weakened pulsation. Head CT shows no abnormalities. The most likely diagnosis is: