**Core Concept**
Xerostomia and xerophthalmia refer to dry mouth and dry eyes, respectively. Both conditions are manifestations of hyposecretion of saliva and tears, primarily due to the lack of functioning acinar cells in the salivary and lacrimal glands.

**Why the Correct Answer is Right**
The correct answer is related to the underlying pathophysiology of these conditions. Xerostomia and xerophthalmia are classic symptoms of Sjögren's syndrome, an autoimmune disease characterized by lymphocytic infiltration and destruction of the exocrine glands, particularly the salivary and lacrimal glands. The autoimmune response is mediated by CD4+ T cells and B cells, leading to the production of autoantibodies against the glandular epithelial cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting, but it is incorrect because while radiation therapy can cause xerostomia, it is not a primary symptom of the condition it causes.
**Option B:** This option is incorrect because while dehydration can cause dry mouth, it is not a primary symptom of the condition it causes, and it does not explain the underlying pathophysiology of xerostomia and xerophthalmia.
**Option C:** This option is incorrect because while diabetes mellitus can cause dry mouth, it is not the primary condition associated with xerostomia and xerophthalmia.

**Clinical Pearl / High-Yield Fact**
Sjögren's syndrome is a classic example of an autoimmune disease that affects multiple organ systems, and it is often associated with other autoimmune conditions such as rheumatoid arthritis and lupus.

**Correct Answer:** D. Sjögren's syndrome