B-vitamin deficiency questions are some of the most “pattern-recognition heavy” on Step 1/2: you’re expected to match a clinical vignette + a key lab clue to the missing cofactor in seconds. This post is a quick-hit, shareable reference with a comparison table, mnemonic-style hooks, and the highest-yield associations you’ll actually see on exams.
How to use this (fast)
When you get a vignette:
- Identify the big syndrome (e.g., dermatitis/diarrhea/dementia, neuropathy, megaloblastic anemia, bleeding).
- Look for risk factor (alcohol use, isoniazid, veganism, raw eggs, malabsorption).
- Confirm with signature clue (elevated homocysteine, methylmalonic acid, corkscrew hair, pellagra triad).
The “B’s” at a glance: what they do
B vitamins mostly act as enzyme cofactors in energy metabolism and synthesis pathways. Deficiency tends to show up in:
- High-turnover tissues (bone marrow → anemia)
- Nervous system (neuropathy, cognitive changes)
- Skin/GI (dermatitis, diarrhea)
Comparison table: B-vitamin deficiencies (high-yield)
| Vitamin (Name) | Major cofactor roles (Step-relevant) | Classic deficiency findings | One-liner mnemonic / visual hook | High-yield risk factors / causes | Testable lab clue(s) |
|---|---|---|---|---|---|
| B1 (Thiamine) | Cofactor for PDH, α-KG dehydrogenase, BCKD, transketolase (PPP) | Wernicke-Korsakoff: confusion, ophthalmoplegia, ataxia; memory issues/confabulation. Dry beriberi (peripheral neuropathy), wet beriberi (high-output CHF) | “Thiamine powers the brain’s fuel gauge” → low B1 = neuro + heart failure | Alcohol use disorder, malnutrition, bariatric surgery, refeeding | ↓RBC transketolase activity; lactic acidosis can occur (impaired PDH) |
| B2 (Riboflavin) | Precursor to FAD/FMN (redox reactions; ETC) | Cheilosis, angular stomatitis, glossitis, corneal vascularization | “B2 keeps lips & tongue ‘lit’” | Poor diet, malabsorption | Nonspecific—think mucocutaneous findings |
| B3 (Niacin) | Precursor to NAD⁺/NADP⁺ | Pellagra: 3 D’s (Dermatitis, Diarrhea, Dementia) ± death | “Niacin = ‘3 D’s’” (picture a sunburned brain with diarrhea) | Hartnup disease, carcinoid syndrome (tryptophan shunted to serotonin), alcoholism, malnutrition, isoniazid | Can be from ↓tryptophan availability; may coexist with other deficiencies |
| B5 (Pantothenic acid) | Component of CoA and fatty acid synthase | Rare; dermatitis, enteritis, alopecia, ± adrenal insufficiency; burning feet syndrome | “Pantothenic = ‘Pan’ = in everything” → deficiency is rare | Severe malnutrition | Typically clinical diagnosis; uncommon on exams |
| B6 (Pyridoxine) | Cofactor for transamination, decarboxylation (neurotransmitters), glycogen phosphorylase, heme synthesis (ALA synthase), cystathionine synthase | Peripheral neuropathy, seizures/irritability, cheilosis; sideroblastic anemia | “B6 builds blood + brain chemicals” | Isoniazid, hydralazine, penicillamine, OCPs, alcoholism | May see ↑homocysteine (due to impaired cystathionine synthase) |
| B7 (Biotin) | Cofactor for carboxylases: pyruvate → OAA, acetyl-CoA → malonyl-CoA, propionyl-CoA → methylmalonyl-CoA | Dermatitis, alopecia, enteritis | “Biotin = Beauty vitamin” (hair/skin) | Raw egg whites (avidin binds biotin), long-term antibiotics | Carboxylase-related issues; classically hair loss + rash |
| B9 (Folate) | THF for 1-carbon transfers (DNA/RNA synthesis); homocysteine → methionine (with B12) | Megaloblastic anemia, glossitis; neural tube defects in fetus | “Folate = Fetal neural tube + Fast-dividing cells” | Poor diet, alcoholism; methotrexate, TMP-SMX, phenytoin; pregnancy | ↑Homocysteine, normal methylmalonic acid |
| B12 (Cobalamin) | Cofactor for methionine synthase and methylmalonyl-CoA mutase | Megaloblastic anemia + neurologic deficits (subacute combined degeneration: dorsal columns/corticospinal tracts) | “B12 = Brain + Bone marrow” | Pernicious anemia (anti–intrinsic factor), ileal disease/resection (Crohn), Diphyllobothrium latum, veganism | ↑Homocysteine + ↑Methylmalonic acid; can see hypersegmented neutrophils |
The ultra-high-yield discriminator: folate vs B12
If the question is basically “megaloblastic anemia—now pick the vitamin,” your fastest move is:
| Feature | Folate (B9) | B12 (Cobalamin) |
|---|---|---|
| Neuro symptoms | No | Yes (posterior column + corticospinal signs) |
| Homocysteine | ↑ | ↑ |
| Methylmalonic acid | Normal | ↑ |
| Common cause | Poor diet, alcohol, MTX/TMP | Pernicious anemia, ileal disease, veganism |
Exam trap: Giving folate can improve anemia in B12 deficiency but does not fix neurologic injury.
Rapid-fire “visual hooks” you can recall under time pressure
B1 (Thiamine): “4 enzymes + 2 syndromes”
- Enzymes: PDH, α-KG DH, BCKD, transketolase
- Syndromes: Wernicke-Korsakoff, beriberi
- Vignette feel: alcohol use + confusion/ataxia/ophthalmoplegia → give thiamine before glucose.
B3 (Niacin): “Pellagra poster”
Imagine a sun-exposed rash in a collar pattern (Casal necklace) + diarrhea + confusion.
- Think: carcinoid (tryptophan diverted) or Hartnup (can’t absorb neutral AAs).
B6 (Pyridoxine): “Isoniazid steals B6”
- INH → functional B6 deficiency → neuropathy + sideroblastic anemia (impaired heme synthesis).
- USMLE loves “TB treatment + tingling hands” → give pyridoxine.
B7 (Biotin): “Raw eggs wreck hair”
- Raw egg whites (avidin) bind biotin → alopecia + dermatitis.
B9/B12: “Homocysteine is not enough”
- Both raise homocysteine; only B12 raises methylmalonic acid and causes neuro deficits.
Quick clinical associations (Step-style)
- Alcohol use disorder → think B1, B9, ± B6
- Bariatric surgery / malabsorption → B1 early, B12 later
- Isoniazid → B6
- Vegan + neuropathy → B12
- Pregnancy + no prenatal vitamins → B9 (neural tube defects)
10-second self-quiz (to cement patterns)
- Confusion + ophthalmoplegia + ataxia in an alcoholic → B1 deficiency
- Dermatitis + diarrhea + dementia → B3 deficiency
- Isoniazid user with neuropathy + microcytic anemia with ring sideroblasts → B6 deficiency
- Megaloblastic anemia + ↑methylmalonic acid + paresthesias → B12 deficiency
- Megaloblastic anemia + normal methylmalonic acid in pregnancy → B9 deficiency
- Alopecia + dermatitis in someone who drinks raw eggs → B7 deficiency