🧠 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

🧠 Biotin Deficiency for USMLE Step 1: Carboxylation Reactions, Causes, and Clinical Features

πŸ”¬ 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

🧠 Mnemonic: β€œABC enzymes need Biotin”

  • Acetyl-CoA carboxylase

  • Biotin

  • Carboxylases (pyruvate & propionyl-CoA)

🍳 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

🧠 Step 1 Clue: "Raw eggs = risk for biotin deficiency"

πŸ‘©β€βš•οΈ 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

πŸ”¬ Labs & Diagnosis

  • ↑ Organic acids (propionic acid, lactic acid)

  • Metabolic acidosis

  • Normal B12 (distinguishes from methylmalonic acidemia)

  • Often clinical diagnosis unless inherited

πŸ’Š Treatment

  • Oral biotin supplementation (5–10 mg/day)

  • Treat underlying causes (stop raw eggs, adjust TPN)

  • For genetic disorders β†’ lifelong biotin therapy

🧠 Mnemonics to Remember

β€œBiotin is the COβ‚‚ carry-on”

β†’ Carries carbon in carboxylation

β€œEggs rob your head”

β†’ Raw eggs bind biotin β†’ hair loss, mental fog

⚠️ Step 1 Clinical Clue

A bodybuilder with a raw egg-heavy diet presents with alopecia, red skin rash near the mouth, and fatigue. What’s the diagnosis?

βœ… Answer: Biotin (Vitamin B7) deficiency
βœ… Mechanism: Avidin in raw eggs binds biotin β†’ cofactor loss

🎯 Call-To-Action

Want to lock in vitamin functions + cofactor enzymes with KOTC-style visuals?

🧠 Check out the KOTC Vitamin Biochem Library + Daily Diagram Drill:
πŸ”— https://kingofthecurve.org/trial-sessions

πŸ“˜ Or unlock lifetime access for full Qbank + visual learning:
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