A 3-year-old boy from rural Maharashtra presents with recurrent episodes of hypoketotic hypoglycemia, muscle weakness, and hepatomegaly. Serum carnitine is low. Which enzyme deficiency is the most common cause of primary carnitine deficiency in children?

Explanation

## Primary Carnitine Deficiency: Etiology and Pathophysiology **Key Point:** Primary carnitine deficiency is most commonly caused by mutations in the *SLC22A5* gene, which encodes the organic cation transporter 2 (OCTN2), responsible for renal reabsorption and intestinal absorption of carnitine. ### Genetic Basis and Frequency | Defect | Gene | Protein | Inheritance | Frequency | Pathophysiology | |--------|------|---------|-------------|-----------|------------------| | **Primary carnitine deficiency** | *SLC22A5* | OCTN2 transporter | Autosomal recessive | Most common | ↓ Carnitine reabsorption → systemic deficiency | | CPT I deficiency | *CPT1A* | CPT I (hepatic) | Autosomal recessive | Rare | ↓ Carnitine shuttle entry; hypoketotic hypoglycemia | | CPT II deficiency | *CPT2* | CPT II (inner membrane) | Autosomal recessive | Rare | ↓ Acyl-carnitine exit; muscle pain, rhabdomyolysis | | ACAD deficiency | *ACAD9*, *ACAD10* | Acyl-CoA dehydrogenase | Autosomal recessive | Rare | ↓ β-oxidation initiation | ### Why OCTN2 (Organic Cation Transporter 2)? 1. **Renal reabsorption:** OCTN2 is the primary transporter for carnitine reabsorption in the proximal tubule; mutations cause massive urinary losses. 2. **Intestinal absorption:** OCTN2 also mediates carnitine uptake in the small intestine; defects reduce dietary carnitine bioavailability. 3. **Systemic consequence:** Loss of both reabsorption and absorption pathways leads to severe, generalized carnitine depletion affecting all tissues (heart, skeletal muscle, liver). 4. **Clinical presentation:** Hypoketotic hypoglycemia (impaired mitochondrial β-oxidation), cardiomyopathy, hepatic encephalopathy, and muscle weakness. **High-Yield:** Primary carnitine deficiency is an autosomal recessive disorder caused by *SLC22A5* mutations and is the most common inherited carnitine disorder; it is treatable with oral L-carnitine supplementation. **Mnemonic:** **OCTN2 = Organic Cation TraNsporter 2** — the gatekeeper of carnitine homeostasis; loss of this transporter = loss of carnitine. **Clinical Pearl:** The hypoketotic hypoglycemia in primary carnitine deficiency is pathognomonic: despite severe hypoglycemia, ketone bodies are NOT elevated because the block in carnitine-dependent fatty acid oxidation prevents ketogenesis. This distinguishes it from other causes of hypoglycemia. ### Differential Diagnosis: Why Not the Others? ```mermaid flowchart TD A[Carnitine Deficiency Suspected]:::outcome --> B{Clinical Presentation?}:::decision B -->|Hypoketotic hypoglycemia + hepatomegaly + low carnitine| C[Primary Carnitine Deficiency]:::action C --> D[OCTN2 mutation - SLC22A5]:::outcome B -->|Muscle pain + rhabdomyolysis + normal carnitine| E[CPT II Deficiency]:::action B -->|Hypoketotic hypoglycemia + normal carnitine| F[CPT I Deficiency]:::action B -->|Elevated acyl-carnitines| G[ACAD Deficiency]:::action ```