A 32-year-old man from rural Maharashtra presents with a 3-week history of polyuria, polydipsia, and weight loss of 4 kg. He reports no prior diabetes in his family. On examination, he appears lean (BMI 21 kg/m²) with dry mucous membranes. Blood glucose is 380 mg/dL, venous pH 7.28, HCO₃⁻ 14 mEq/L, and serum ketones are positive. C-peptide is undetectable. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of acute onset (3 weeks), lean body habitus, severe hyperglycaemia, metabolic acidosis with positive ketones, and undetectable C-peptide is pathognomonic for Type 1 diabetes presenting as diabetic ketoacidosis (DKA). ## Diagnostic Features Supporting Type 1 DM | Feature | Type 1 DM (DKA) | Type 2 DM (HHS) | MODY | Secondary | |---------|-----------------|-----------------|------|----------| | **Age of onset** | Young (< 40 yrs) | Usually > 40 yrs | < 25 yrs, family history | Variable | | **BMI** | Lean (< 25) | Obese (> 25) | Lean/normal | Variable | | **Presentation** | Acute (days–weeks) | Insidious (months–years) | Insidious, familial | Depends on cause | | **Ketosis** | Present (DKA) | Absent (HHS) | Absent | Absent | | **C-peptide** | Undetectable | Normal/elevated | Normal | Normal/low | | **pH** | < 7.30 (acidosis) | > 7.30 (no acidosis) | Normal | Normal | | **HCO₃⁻** | < 15 mEq/L | > 15 mEq/L | Normal | Normal | **High-Yield:** Undetectable C-peptide indicates absolute insulin deficiency — the hallmark of Type 1 DM. This distinguishes it from Type 2 DM (where C-peptide is normal or elevated) and secondary causes. ## Pathophysiology of DKA in Type 1 DM 1. Absolute insulin deficiency → uncontrolled lipolysis 2. Free fatty acids → ketone body production (acetoacetate, β-hydroxybutyrate) 3. Ketone accumulation → metabolic acidosis (pH < 7.30, HCO₃⁻ < 15) 4. Osmotic diuresis from hyperglycaemia → polyuria, dehydration 5. Respiratory compensation (Kussmaul breathing) → ↓ PaCO₂ **Clinical Pearl:** The acute presentation in a lean young adult with no family history suggests autoimmune destruction of pancreatic β-cells, typical of Type 1 DM. Environmental triggers (viral infection, stress) often precede onset. **Mnemonic for DKA vs HHS:** **KETO** — Type 1 (Ketoacidosis), **HYPERos** — Type 2 (Hyperosmolar). Type 1 = Ketones + Acidosis; Type 2 = No ketones + No acidosis (or mild).