**Core Concept**
Conjunctival xerosis refers to the dryness and lack of moisture on the conjunctiva, the thin membrane covering the white part of the eye and the inner side of the eyelids. It is often associated with various systemic conditions that affect the body's ability to produce tears or maintain moisture in the eyes.

**Why the Correct Answer is Right**
Conjunctival xerosis is commonly seen in Sjögren's syndrome, an autoimmune disorder characterized by lymphocytic infiltration and destruction of exocrine glands, particularly the salivary and lacrimal glands. This leads to decreased tear production and subsequent dryness of the eyes. The reduced tear production is due to the destruction of the lacrimal gland, which is responsible for producing tears.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because conjunctival xerosis is not typically associated with diabetes mellitus. While diabetes can cause dry eye syndrome, it is not the primary cause of conjunctival xerosis.
**Option B:** This option is incorrect because conjunctival xerosis is not a characteristic finding in rheumatoid arthritis. Although rheumatoid arthritis is an autoimmune disorder, it primarily affects the joints and does not typically cause dryness of the eyes.
**Option C:** This option is incorrect because conjunctival xerosis is not typically associated with hypothyroidism. While hypothyroidism can cause dry skin and mucous membranes, it is not the primary cause of conjunctival xerosis.

**Clinical Pearl / High-Yield Fact**
Sjögren's syndrome is often associated with other autoimmune disorders, such as rheumatoid arthritis and lupus, and can cause dryness of the mouth, skin, and respiratory tract in addition to the eyes.

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