why rrect: high-resolution color doppler ultrasound of the temporal and axillary arteries is the preferred initial imaging test for suspected giant cell arteritis in many guideline-based pathways. ultrasound can demonstrate a concentric hypoechoic "halo" sign and stenosis or occlusion; when performed promptly (ideally within days to 2 weeks of symptom onset and after starting glucocorticoids) it has good sensitivity and specificity for cranial and large-vessel involvement, is noninvasive, readily available in outpatient settings, and can guide management and the need for further testing. including the axillary arteries increases detection of large-vessel disease.

why the other answers are incorrect: immediate unilateral temporal artery biopsy (a) has historically been the gold standard but is invasive, may miss segmental disease, and is often deferred when temporal artery ultrasound is diagnostic. biopsy is appropriate when ultrasound is unavailable or inconclusive. contrast-enhanced mr angiography (b) can identify large-vessel involvement but is less sensitive than targeted ultrasound for superficial temporal artery changes, more costly, and not required as the first-line test in an outpatient evaluation. 18f-fdg pet-ct (d) is useful to detect large-vessel vasculitis but is less useful for cranial arteritis, involves radiation, and is not the preferred initial test in an ambulatory patient with prominent temporal symptoms when ultrasound is available.