A 35-year-old woman presents with polyuria, polydipsia, and weight loss over 2 weeks. Laboratory investigations reveal fasting blood glucose 280 mg/dL, random blood glucose 320 mg/dL, and HbA1c 9.2%. Serum C-peptide is undetectable, and anti-GAD antibodies are positive. What is the most common cause of diabetes mellitus in this patient?

Explanation

## Diagnosis: Type 1 Diabetes Mellitus ### Clinical Presentation The patient presents with classic acute-onset diabetes (polyuria, polydipsia, weight loss) in a young adult with rapid progression over weeks. ### Diagnostic Clues - **Undetectable C-peptide**: indicates severe beta-cell dysfunction or absence - **Anti-GAD antibodies positive**: pathognomonic for autoimmune beta-cell destruction - **High HbA1c with acute symptoms**: suggests recent loss of insulin secretion - **Young age at presentation**: Type 1 DM typically presents in childhood/young adulthood ### Pathophysiology of Type 1 DM **Key Point:** Type 1 diabetes results from autoimmune destruction of insulin-producing beta cells in the islets of Langerhans, leading to absolute insulin deficiency. 1. **Genetic predisposition**: HLA-DR3/DR4 haplotypes 2. **Environmental trigger**: viral infection (enterovirus, rotavirus) or molecular mimicry 3. **Autoimmune attack**: T-cell mediated destruction of beta cells 4. **Progressive beta-cell loss**: 80–90% destruction before hyperglycemia manifests 5. **Absolute insulin deficiency**: requires insulin replacement therapy ### Autoimmune Markers | Marker | Prevalence in Type 1 DM | Significance | |--------|-------------------------|---------------| | Anti-GAD65 | 70–80% | Most common; present at diagnosis | | Anti-IA2 | 50–60% | Indicates active autoimmunity | | Anti-ZnT8 | 50–60% | Zinc transporter antibody | | ICA (Islet Cell Antibody) | 70–80% | Broad autoimmune marker | | Anti-insulin | 40–50% | Often transient | **High-Yield:** The presence of **any one autoimmune marker** (especially anti-GAD) confirms autoimmune beta-cell destruction as the cause of diabetes. ### Distinction from Type 2 DM | Feature | Type 1 DM | Type 2 DM | |---------|-----------|----------| | **Onset** | Acute (days to weeks) | Insidious (months to years) | | **Age** | Usually <30 years | Usually >40 years | | **BMI** | Normal or low | Often overweight/obese | | **C-peptide** | Low/undetectable | Normal or elevated | | **Autoimmune markers** | Positive | Negative | | **Insulin requirement** | Absolute, from onset | Not required initially | | **Pathophysiology** | Beta-cell destruction | Insulin resistance + relative deficiency | **Clinical Pearl:** Undetectable C-peptide in the presence of hyperglycemia is diagnostic of Type 1 DM; C-peptide is co-secreted with insulin in a 1:1 molar ratio and reflects endogenous insulin production. **Mnemonic: GADA** — **G**AD antibodies, **A**utoimmune, **D**estruction, **A**bsolute insulin deficiency.