A 32-year-old woman presents with acute onset severe thyroid pain, fever, and dysphagia. On examination, the thyroid is tender and enlarged. TSH is suppressed (0.1 mIU/L) with elevated free T4 (18 pg/dL). Thyroid peroxidase (TPO) antibodies are negative. Which investigation is most specific for confirming the diagnosis of acute suppurative thyroiditis?