A 28-year-old woman with type 1 diabetes mellitus presents to the emergency department with a 2-day history of polyuria, polydipsia, and progressive lethargy. She admits to missing insulin injections for 3 days. On examination, she is tachypneic (RR 28/min), blood pressure 110/70 mmHg, and has fruity-smelling breath. Laboratory investigations reveal: blood glucose 580 mg/dL, arterial pH 7.18, HCO₃⁻ 12 mEq/L, and serum ketones strongly positive. Which of the following best explains the biochemical mechanism underlying the elevated ketone bodies in this patient?

Explanation

## Pathophysiology of Diabetic Ketoacidosis (DKA) ### The Biochemical Cascade **Key Point:** In DKA, insulin deficiency removes the brake on lipolysis, causing uncontrolled fatty acid mobilization and β-oxidation that overwhelms ketone body utilization. ### Step-by-Step Mechanism 1. **Insulin deficiency** → Loss of inhibition on hormone-sensitive lipase (HSL) in adipose tissue 2. **Increased HSL activity** → Massive release of free fatty acids (FFAs) from triglyceride stores 3. **Elevated FFAs** → Transport to liver via albumin; CPT-I activity is actually *increased* (not decreased) because malonyl-CoA levels fall when ACC is inhibited by AMP-activated protein kinase (AMPK) during energy deficit 4. **Mitochondrial β-oxidation** → Excessive acetyl-CoA production from FFA catabolism 5. **TCA cycle saturation** → The Krebs cycle cannot oxidize all incoming acetyl-CoA; excess acetyl-CoA is shunted to ketogenesis 6. **Ketone body accumulation** → β-hydroxybutyrate and acetoacetate accumulate faster than peripheral tissues can utilize them, causing metabolic acidosis ### Why Ketogenesis Accelerates **High-Yield:** The ratio of acetyl-CoA to CoA and NADH to NAD⁺ both increase, driving the equilibrium toward ketone body synthesis via HMG-CoA synthase in the liver mitochondria. ### Clinical Correlation **Clinical Pearl:** The fruity breath (acetone), Kussmaul respirations (compensatory hyperventilation for metabolic acidosis), and positive serum ketones are the hallmark triad of DKA. The strong positive ketones indicate severe ketosis. ### Why Option 2 Is Wrong ~~Decreased CPT-I activity~~ — In DKA, CPT-I activity is actually *increased* because malonyl-CoA (an allosteric inhibitor of CPT-I) is depleted when ACC is inhibited. This allows more fatty acids to enter mitochondria for oxidation, not fewer. ```mermaid flowchart TD A[Insulin Deficiency]:::urgent --> B[Loss of HSL Inhibition]:::action B --> C[Increased Lipolysis in Adipose Tissue]:::action C --> D[Elevated Free Fatty Acids]:::outcome D --> E[Increased CPT-I Activity<br/>malonyl-CoA ↓]:::action E --> F[Enhanced Mitochondrial β-Oxidation]:::action F --> G[Excess Acetyl-CoA]:::outcome G --> H{TCA Cycle Capacity?}:::decision H -->|Exceeded| I[Shunt to Ketogenesis]:::action I --> J[Ketone Body Accumulation]:::urgent J --> K[Metabolic Acidosis]:::urgent ```