**Core Concept**
Monogenic diabetes mellitus is a rare form of diabetes caused by mutations in a single gene, affecting insulin secretion or action. This contrasts with polygenic forms, where multiple genetic and environmental factors contribute to disease development.

**Why the Correct Answer is Right**
Maturity Onset Diabetes of the Young (MODY) is a hereditary form of diabetes characterized by an autosomal dominant pattern of inheritance. It is caused by mutations in genes involved in insulin secretion, such as the HNF1A, HNF4A, or GCK genes. These mutations lead to impaired insulin production and beta-cell dysfunction, resulting in hyperglycemia. MODY typically presents in young adulthood and is often diagnosed after the age of 20, hence the name.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulin Dependent Diabetes Mellitus (IDDM), formerly known as Type 1 Diabetes, is an autoimmune disease where the immune system attacks and destroys pancreatic beta-cells, leading to absolute insulin deficiency. While IDDM has a strong genetic component, it is not caused by a single gene mutation.

**Option B:** Non-insulin Dependent Diabetes Mellitus (NIDDM), also known as Type 2 Diabetes, is a polygenic disorder characterized by insulin resistance and impaired insulin secretion. It is influenced by a combination of genetic and environmental factors, including obesity, physical inactivity, and a high-energy diet.

**Option C:** Latent Autoimmune Diabetes in Adults (LADA) is a form of autoimmune diabetes that presents in adulthood, often with a slower progression to insulin deficiency compared to IDDM. While LADA has some genetic components, it is not a monogenic disorder.

**Clinical Pearl / High-Yield Fact**
MODY patients often have a strong family history of diabetes, and genetic testing can identify the underlying causative mutation. Early diagnosis and management of MODY are crucial to prevent long-term complications and improve quality of life.

✓ Correct Answer: D. Maturity Onset Diabetes of the Young (MODY).