# Clinical Cases: Lipid Metabolism

## Case 1: Diabetic Ketoacidosis (Uncontrolled Ketogenesis)

### Clinical Image
![DKA Pathophysiology](case_01_image.jpg)
*Source: [Wikipedia - Diabetic ketoacidosis](https://en.wikipedia.org/wiki/Diabetic_ketoacidosis) - Public Domain*

### Case Presentation
A 22-year-old man with type 1 diabetes presents to the emergency department with nausea, vomiting, abdominal pain, and altered mental status. He reports running out of insulin 3 days ago. On examination, he is lethargic but arousable, has dry mucous membranes, deep rapid breathing (Kussmaul respirations), and a fruity odor on his breath. Vital signs show heart rate 120, blood pressure 95/60, respiratory rate 28. Laboratory studies reveal blood glucose 485 mg/dL, pH 7.15, pCO2 18 mmHg, HCO3 8 mEq/L, anion gap 28, serum ketones strongly positive, potassium 5.8 mEq/L (elevated due to acidosis causing cellular shift). Urinalysis shows 3+ ketones and 4+ glucose. He is diagnosed with diabetic ketoacidosis (DKA). The pathophysiology is explained: absolute insulin deficiency combined with elevated glucagon creates a metabolic state mimicking severe starvation. Hormone-sensitive lipase in adipose tissue is maximally activated, flooding the liver with free fatty acids. Hepatic beta-oxidation generates excess acetyl-CoA, but gluconeogenesis diverts oxaloacetate away from the TCA cycle. Unable to enter the cycle, acetyl-CoA is converted to ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone). He is treated with IV insulin, aggressive fluid resuscitation, and potassium replacement.

### Key Learning Points
- In insulin deficiency, unrestrained lipolysis releases fatty acids that undergo hepatic beta-oxidation to acetyl-CoA; simultaneously, gluconeogenesis depletes oxaloacetate, preventing acetyl-CoA from entering the TCA cycle
- Excess acetyl-CoA is diverted to ketogenesis; ketone bodies are moderately strong organic acids that cause metabolic acidosis with a high anion gap
- The fruity breath odor results from acetone (formed by spontaneous decarboxylation of acetoacetate), which is volatile and exhaled; Kussmaul respirations represent respiratory compensation for metabolic acidosis

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## Case 2: Medium-Chain Acyl-CoA Dehydrogenase (MCAD) Deficiency

### Clinical Image
![MCAD Deficiency Acylcarnitine](case_02_image.jpg)
*Source: [Wikipedia - Medium-chain acyl-CoA dehydrogenase deficiency](https://en.wikipedia.org/wiki/Medium-chain_acyl-CoA_dehydrogenase_deficiency) - CC BY-SA 3.0*

### Case Presentation
A 9-month-old infant is brought to the emergency department after being found unresponsive. He had mild upper respiratory symptoms for 2 days and decreased oral intake. Prior to this, he was a healthy, normally developing child. On examination, he is hypotonic and poorly responsive. Blood glucose is critically low at 25 mg/dL. Surprisingly, serum ketones are inappropriately low (hypoketotic hypoglycemia). Liver function tests show mild transaminase elevation. He is immediately given IV dextrose with rapid improvement in mental status. Given the presentation of hypoketotic hypoglycemia triggered by fasting during illness, a fatty acid oxidation disorder is suspected. Newborn screening results are retrieved and show elevated C8 (octanoylcarnitine) on his blood spot, which had been flagged but the family was not contacted. Confirmatory testing shows elevated medium-chain acylcarnitines and reduced MCAD enzyme activity. Genetic testing confirms homozygous mutations in ACADM. The family is extensively counseled on avoidance of fasting, emergency protocols during illness (cornstarch, frequent feeding, early IV dextrose), and wearing a medical alert bracelet. With proper management, prognosis is excellent.

### Key Learning Points
- MCAD deficiency is the most common inherited fatty acid oxidation disorder; affected individuals cannot complete beta-oxidation of medium-chain fatty acids, blocking ketone body production during fasting
- Hypoketotic hypoglycemia is the hallmark: during fasting, glucose falls as hepatic glycogen is depleted, but ketones fail to rise because beta-oxidation is blocked, leaving the brain without alternative fuel
- Elevated octanoylcarnitine (C8) on acylcarnitine profile is diagnostic; MCAD deficiency is included in newborn screening programs because early diagnosis and fasting avoidance prevent life-threatening episodes

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## Case 3: Carnitine Palmitoyltransferase II (CPT-II) Deficiency

### Clinical Image
![Rhabdomyolysis](case_03_image.jpg)
*Source: [Wikipedia - Rhabdomyolysis](https://en.wikipedia.org/wiki/Rhabdomyolysis) - CC BY-SA 3.0*

### Case Presentation
A 25-year-old man presents to the emergency department with severe muscle pain and dark brown urine after completing a marathon. He reports this is his third episode of similar symptoms, with previous episodes occurring after prolonged exercise and once during a fasting religious observance. Physical examination reveals diffuse muscle tenderness. Urinalysis shows 3+ blood but no red blood cells on microscopy (myoglobinuria). Laboratory studies reveal CK 85,000 U/L (markedly elevated), creatinine 2.4 mg/dL (acute kidney injury from myoglobin nephrotoxicity), and elevated transaminases. He is diagnosed with rhabdomyolysis. Given his recurrent episodes triggered by prolonged exercise and fasting, a metabolic myopathy is suspected. Acylcarnitine profile shows elevated long-chain acylcarnitines. Genetic testing confirms compound heterozygous mutations in CPT2, consistent with the adult myopathic form of CPT-II deficiency. The carnitine shuttle is essential for transporting long-chain fatty acids into mitochondria; during prolonged exercise when fatty acids become the primary fuel, impaired import leads to energy failure and muscle breakdown. He is counseled on avoiding prolonged exercise, maintaining carbohydrate intake during physical activity, and pre-treatment hydration strategies.

### Key Learning Points
- CPT-II deficiency impairs transport of long-chain fatty acids into mitochondria via the carnitine shuttle; the adult myopathic form causes recurrent rhabdomyolysis triggered by prolonged exercise, fasting, cold, or infection
- During sustained exercise, muscles increasingly rely on fatty acid oxidation; when this is impaired, ATP depletion causes muscle breakdown (rhabdomyolysis) with release of myoglobin and CK
- Myoglobinuria can cause acute kidney injury through tubular obstruction and direct nephrotoxicity; treatment includes aggressive IV fluid resuscitation to maintain urine output