**Core Concept**
Sjögren's syndrome is an autoimmune disorder characterized by lymphocytic infiltration and destruction of exocrine glands, particularly the salivary and lacrimal glands. This leads to symptoms such as dry mouth, dry eyes, and difficulty swallowing due to reduced saliva production.

**Why the Correct Answer is Right**
The definitive diagnosis of Sjögren's syndrome is made by demonstrating focal lymphocytic sialadenitis and/or focal lymphocytic lacrimal gland inflammation on histopathological examination of minor salivary gland biopsy samples. This test is considered the gold standard for diagnosis, as it directly shows the characteristic lymphocytic infiltration of the gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood tests for anti-SSA/Ro and anti-SSB/La antibodies are useful for supporting the diagnosis, but they are not definitive, as these antibodies can be present in other autoimmune diseases as well.
**Option B:** Schirmer's test measures tear production but does not diagnose Sjögren's syndrome, as it can be abnormal in other conditions such as dry eye syndrome.
**Option C:** Imaging studies like sialography or ultrasound may show abnormalities suggestive of Sjögren's syndrome, but they are not definitive, as these findings can be seen in other conditions as well.

**Clinical Pearl / High-Yield Fact**
Minor salivary gland biopsy is a sensitive and specific test for diagnosing Sjögren's syndrome, and it's often performed on the lower lip.

**Correct Answer:** D. Minor salivary gland biopsy.