**Question:** The investigation of choice for diagnosis of Giant cell Aeritis is

A. MRI
B. CT scan
C. Biopsy
D. Ultrasound

**Correct Answer:** C. Biopsy

**Core Concept:** Giant cell arteritis (GCAs) is a form of vasculitis, characterized by inflammation of the medium and large-sized arteries, predominantly affecting the temporal artery in older adults. The clinical presentation often includes headache, scalp tenderness, jaw claudication, and visual disturbances. GCA is a clinical diagnosis, and the role of investigations is to confirm the diagnosis and rule out other conditions.

**Why the Correct Answer is Right:** A biopsy is the gold standard investigation for confirming the diagnosis of GCA. It involves taking a small tissue sample (biopsy) from the affected artery, typically the temporal artery, to examine the presence of giant cells (histiocytes) and other inflammation markers under a microscope. This confirms the diagnosis of GCA and differentiates it from other vasculitides, such as Takayasu arteritis or polyarteritis nodosa.

**Why Each Wrong Option is Incorrect:**

A. MRI (Magnetic Resonance Imaging) and CT (Computed Tomography) scans: These imaging modalities may demonstrate arterial wall thickening and enhancement, but they lack the sensitivity to confirm GCA diagnosis. In some cases, they may even suggest a diagnosis of Takayasu arteritis or fibromuscular dysplasia.

B. CT scan: Similar to MRI, CT scan may demonstrate arterial wall thickening but lacks the sensitivity for GCA diagnosis.

D. Ultrasound: Ultrasound is a non-invasive modality often used for temporal artery evaluation. However, it does not have a high enough sensitivity to definitively confirm GCA diagnosis. A biopsy remains essential for diagnosis.

**Clinical Pearl:** Biopsy is crucial for confirming the diagnosis of GCA as it directly demonstrates the presence of giant cells and inflammation in the arterial wall. Clinical examination, including temporal artery palpation, is essential for suspecting GCA, but biopsy confirms the diagnosis. In cases where a biopsy is not feasible or contraindicated, a combination of clinical findings, laboratory tests, and imaging studies may be used for a probable diagnosis of GCA. However, the definitive diagnosis still relies on biopsy.