π§ Biotin Deficiency for USMLE Step 1: Carboxylation Reactions, Causes, and Clinical Features
Biotin (Vitamin B7) is a water-soluble vitamin crucial for carboxylation reactions. Though rare, its deficiency shows up in:
Clinical cases involving dietary excesses or genetic conditions
Enzyme-heavy Step 1 questions
Dermatologic and neurologic presentations
π¬ Biotin as a Carboxylase Cofactor
Biotin functions as a coenzyme for carboxylation enzymes, which add COβ to substrates.
Enzyme | Pathway | Reaction |
---|---|---|
Pyruvate carboxylase | Gluconeogenesis | Pyruvate β oxaloacetate |
Acetyl-CoA carboxylase | Fatty acid synthesis | Acetyl-CoA β malonyl-CoA |
Propionyl-CoA carboxylase | Odd-chain FA metabolism | Propionyl-CoA β methylmalonyl-CoA |
π§ Mnemonic: βABC enzymes need Biotinβ
Acetyl-CoA carboxylase
Biotin
Carboxylases (pyruvate & propionyl-CoA)
π³ Causes of Biotin Deficiency
Cause | Mechanism |
---|---|
Raw egg white consumption | Avidin binds biotin, blocking absorption |
Antibiotic use | Disrupts gut flora that synthesize biotin |
Inborn errors | Holocarboxylase synthase/biotinidase deficiency |
Total parenteral nutrition (TPN) | Missing biotin supplementation |
π§ Step 1 Clue: "Raw eggs = risk for biotin deficiency"
π©ββοΈ Clinical Presentation
System | Findings |
---|---|
Skin | Scaly rash, seborrheic dermatitis (esp. around eyes/nose/mouth) |
Hair | Alopecia (hair loss) |
Neurologic | Depression, hallucinations, paresthesia |
Metabolic | Lactic acidosis, mild ketoacidosis |
π¬ Labs & Diagnosis
β Organic acids (propionic acid, lactic acid)
Metabolic acidosis
Normal B12 (distinguishes from methylmalonic acidemia)
Often clinical diagnosis unless inherited
π Treatment
Oral biotin supplementation (5β10 mg/day)
Treat underlying causes (stop raw eggs, adjust TPN)
For genetic disorders β lifelong biotin therapy
π§ Mnemonics to Remember
βBiotin is the COβ carry-onβ
β Carries carbon in carboxylation
βEggs rob your headβ
β Raw eggs bind biotin β hair loss, mental fog
β οΈ Step 1 Clinical Clue
A bodybuilder with a raw egg-heavy diet presents with alopecia, red skin rash near the mouth, and fatigue. Whatβs the diagnosis?
β
Answer: Biotin (Vitamin B7) deficiency
β
Mechanism: Avidin in raw eggs binds biotin β cofactor loss
π― Call-To-Action
Want to lock in vitamin functions + cofactor enzymes with KOTC-style visuals?
π§ Check out the KOTC Vitamin Biochem Library + Daily Diagram Drill:
π https://kingofthecurve.org/trial-sessions
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