**Core Concept**
Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia (high blood sugar levels), which can be caused by insulin deficiency, insulin resistance, or both. Certain genetic syndromes are associated with an increased risk of developing diabetes mellitus due to underlying pancreatic or hormonal abnormalities.

**Why the Correct Answer is Right**
Pendred syndrome is a genetic disorder characterized by congenital bilateral sensorineural hearing loss, goiter (enlargement of the thyroid gland), and normal intelligence. It is caused by mutations in the SLC26A4 gene, which encodes a protein involved in iodide transport in the thyroid gland, but it is not associated with an increased risk of diabetes mellitus. In contrast, the other options (Down's syndrome, Turner's syndrome, and Klinefelter's syndrome) are all associated with an increased risk of developing diabetes mellitus due to underlying pancreatic or hormonal abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** Pendred syndrome is primarily associated with thyroid dysfunction and hearing loss, not diabetes mellitus.
**Option B:** Down's syndrome is associated with an increased risk of developing diabetes mellitus due to pancreatic islet cell dysfunction and insulin resistance.
**Option C:** Turner's syndrome is associated with an increased risk of developing diabetes mellitus due to pancreatic islet cell dysfunction and insulin resistance.
**Option D:** Klinefelter's syndrome is associated with an increased risk of developing diabetes mellitus due to insulin resistance and pancreatic islet cell dysfunction.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to recognize the increased risk of developing diabetes mellitus in patients with certain genetic syndromes, such as Down's syndrome, Turner's syndrome, and Klinefelter's syndrome, and to monitor their glucose levels and pancreatic function accordingly.

**✓ Correct Answer: A. Pendred syndrome. Pendred syndrome is not associated with diabetes mellitus.**