Source / episode info
- **Episode:**168
- **Title:**Divine Intervention Episode 168 – USMLE Step 1 Rapid Review Series 11 (Integrated).
- **Published:**2019-10-12
- Source:Episode page
One-liner
This integrated review covers B12/folate metabolism (absorption mechanisms, lab abnormalities), neurological syndromes like subacute combined degeneration and anterior spinal artery syndrome, genetic disorders (homocystinuria vs. Marfan syndrome), and key pharmacology principles including folate pathway inhibitors and anti-infective agents.
High-yield summary
- B12 Deficiency: Causes megaloblastic anemia and Subacute Combined Degeneration of the Cord (loss of vibration/proprioception + UMN signs). Elevated labs include methylmalonic acid (MMA) and homocysteine.
- B12 Absorption Pathway: Requires stomach acidity for pepsin activation, which cleaves B12 from dietary protein; subsequent absorption in the terminal ileum requires Intrinsic Factor (IF). PPIs impair this process.
- Homocysteine Elevation: Can be caused by B12 deficiency (impaired Methionine Synthase), Folate deficiency (lack of methyl donor MTHF), or enzyme deficiencies like CBS (Cystathionine Beta-synthase) in homocystinuria.
- Wernicke-Korsakoff Syndrome: Always administer Thiamine first before glucose to prevent exacerbation of neurological symptoms due to impaired Pyruvate Dehydrogenase Complex activity.
- Folate Pathway Drugs: Methotrexate, Trimethoprim-Sulfamethoxazole (TMP-SMX), and Pyrimethamine all inhibit Dihydrofolate Reductase (DHFR). Bone marrow suppression requires rescue with Leucovory (a pyrimidine analog).
Learning objectives
- Differentiate the clinical and biochemical manifestations of B12 deficiency versus folate deficiency.
- Describe the absorption pathway for Vitamin B12 and identify common inhibitors (e.g., PPIs).
- Recognize the neurological syndromes associated with B12 deficiency, including Subacute Combined Degeneration and Anterior Spinal Artery Syndrome.
- Compare the clinical features and inheritance patterns of Homocystinuria, Marfan Syndrome, and Loeys-Dietz Syndrome.
- Understand the metabolic pathways involving homocysteine, methylmalonic acid, and the drugs that inhibit folate metabolism (e.g., MTX, TMP-SMX).
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