Source / episode info

• **Episode:**517
• **Title:**Divine Intervention Episode 517: B3 and The USMLE
• **Published:**2024-03-05
• Source:Episode page

One-liner

Episode 517 provides a comprehensive review of niacin deficiency and pelagra, emphasizing that the USMLE tests descriptive symptoms, and details multiple mechanisms for B3 deficiency including malabsorption (Crohn's), genetic defects (Hartnup, Fanconi, MSUD), drug interference (INH, Chloramphenicol), and excessive consumption pathways (Carcinoid syndrome).

High-yield summary

• Pelagra Triad: The classic triad is Dermatitis (often a rash on the neck/hands), Diarrhea, and Dementia (or confusion/aggression).
• Niacin Synthesis: Niacin (Nicotinic Acid) is synthesized endogenously from Tryptophan via Tryptophan hydroxylase.
• Genetic Defects:
• Hartnup Disease: Impaired reabsorption of non-polar amino acids (e.g., Tryptophan) in the proximal tubule, leading to urinary loss and subsequent niacin deficiency.
• Fanconi Syndrome: Global proximal tubular defect causing generalized inability to reabsorb substances (including bicarbonate and non-polar amino acids), resulting in Type 2 RTA and niacin deficiency.
• Maple Syrup Urine Disease (MSUD): Buildup of branched-chain amino acids (especially Leucine) inhibits a key enzyme necessary for niacin synthesis, causing B3 deficiency symptoms.
• Drug/Toxin Causes: Drugs like Isoniazid (INH) and Chloramphenicol can cause secondary Niacin deficiency by interfering with the Tryptophan -> Niacin conversion pathway.
• Syndromic Causes: Excessive consumption of Tryptophan for other purposes, such as in Carcinoid Syndrome (excessive serotonin production), depletes the pool needed for niacin synthesis.

Learning objectives

• Identify the classic clinical manifestations and pathophysiology of niacin deficiency (Pelagra).
• Differentiate between various causes of malabsorption or metabolic defects leading to B3 deficiency (e.g., Hartnup vs Fanconi).
• Recognize drug-induced deficiencies, specifically those involving Tryptophan metabolism (INH, Chloramphenicol).