Q-Bank Breakdown: Vitamin C (scurvy) — Why Every Answer Choice Matters
Tag: Biochemistry > Vitamins & Cofactors
Vitamin questions love to hide inside “common” clinical presentations—then punish you with distractors that sound plausible. This post breaks down a classic Vitamin C (ascorbic acid) deficiency vignette and, like a real Q-bank explanation, shows why the correct answer is correct and why every other answer choice is wrong.
The Clinical Vignette (Classic Q-Bank Style)
A 54-year-old man with alcohol use disorder and limited food access presents with fatigue, easy bruising, and bleeding gums. He notes “small red-purple spots” around hair follicles on his legs. Exam shows perifollicular hemorrhages and corkscrew hairs. Labs are otherwise unremarkable.
Question: The underlying biochemical defect most directly involves impaired:
A. Hydroxylation of proline and lysine residues in collagen
B. γ-carboxylation of glutamate residues in clotting factors
C. Cross-linking of collagen by lysyl oxidase
D. Conversion of methylmalonyl-CoA to succinyl-CoA
E. Transketolase activity in the pentose phosphate pathway
Correct Answer: A. Hydroxylation of proline and lysine residues in collagen
Why this is the answer
Vitamin C (ascorbic acid) is required to maintain iron in the Fe²⁺ state in prolyl and lysyl hydroxylases, enzymes that hydroxylate proline and lysine on procollagen. These hydroxylations stabilize the collagen triple helix and support normal connective tissue strength.
How it explains the symptoms
Impaired collagen → fragile blood vessels and poor connective tissue integrity, leading to:
- Bleeding gums
- Easy bruising
- Perifollicular hemorrhages
- Corkscrew hairs
- Poor wound healing
- Bone pain (subperiosteal hemorrhage; especially high-yield in children)
High-yield associations
- Risk factors: alcoholism, malnutrition, elderly, restrictive diets, food insecurity, severe mental illness, “tea and toast” diet
- Vitamin C roles (Step-friendly):
- Collagen synthesis (proline/lysine hydroxylation)
- Increases iron absorption (reduces Fe³⁺ → Fe²⁺)
- Dopamine → norepinephrine (dopamine β-hydroxylase cofactor)
Distractor Breakdown: Why the Other Choices Are Wrong (But Tempting)
B. γ-carboxylation of glutamate residues in clotting factors
This is Vitamin K deficiency.
What Vitamin K does:
- Cofactor for γ-glutamyl carboxylase
- Activates clotting factors II, VII, IX, X and proteins C and S
- Enables calcium binding on clotting factors
How it presents:
- Increased PT/INR (often first)
- Bleeding without the hallmark collagen findings (no corkscrew hairs/perifollicular hemorrhage)
Why it’s tempting: bleeding gums/bruising sounds “coagulation-related,” but the vignette is screaming connective tissue.
C. Cross-linking of collagen by lysyl oxidase
This is a copper (Cu²⁺)-dependent enzyme (not vitamin C).
Copper deficiency can impair lysyl oxidase → decreased collagen cross-linking. Classic related pathology:
- Menkes disease (defective copper transport): brittle “kinky” hair, hypotonia, seizures, connective tissue issues
- Ehlers-Danlos variants can also involve collagen processing defects (but not due to vitamin C deficiency specifically)
Why it’s wrong here: scurvy is about hydroxylation, not cross-linking.
D. Conversion of methylmalonyl-CoA to succinyl-CoA
This is Vitamin B12 (cobalamin).
B12 deficiency:
- Methylmalonic acid increases
- Homocysteine increases
- Neurologic symptoms (subacute combined degeneration)
- Megaloblastic anemia (macrocytosis)
Why it’s tempting: “malnutrition/alcohol” can cause multiple deficiencies, but the vignette’s physical findings (corkscrew hairs, perifollicular hemorrhage) are far more specific for vitamin C.
E. Transketolase activity in the pentose phosphate pathway
This is Thiamine (Vitamin B1).
Thiamine deficiency syndromes:
- Wernicke encephalopathy: confusion, ophthalmoplegia, ataxia
- Korsakoff syndrome: memory impairment, confabulation
- Beriberi:
- Dry: peripheral neuropathy
- Wet: high-output heart failure, edema
Why it’s wrong here: the vignette is not neurologic/cardiac failure; it’s connective tissue bleeding and hair changes.
How to Lock This In on Test Day (Pattern Recognition)
Scurvy = “Fragile collagen” clues
- Bleeding gums
- Perifollicular hemorrhage
- Corkscrew hairs
- Poor wound healing
- Easy bruising
One-liner you can recall quickly
Vitamin C deficiency → ↓ proline/lysine hydroxylation → weak collagen → bleeding + corkscrew hairs.
Bonus: Rapid Comparison Table (High-Yield)
| Nutrient | Key Enzyme/Pathway | Classic Presentation | Lab Clue |
|---|---|---|---|
| Vitamin C | Prolyl/lysyl hydroxylation (collagen) | Bleeding gums, corkscrew hairs, perifollicular hemorrhage | Often nonspecific; consider low ascorbate |
| Vitamin K | γ-carboxylation of factors II, VII, IX, X, C, S | Bleeding, ↑ PT/INR | ↑ PT first |
| Copper | Lysyl oxidase (collagen cross-linking) | Menkes: kinky hair, neuro issues | Low copper/ceruloplasmin (varies by cause) |
| B12 | Methionine synthase, methylmalonyl-CoA mutase | Macrocytic anemia + neuro deficits | ↑ MMA, ↑ homocysteine |
| B1 | Transketolase, PDH, α-KGDH | Wernicke-Korsakoff, beriberi | Low thiamine; response to thiamine |
USMLE High-Yield Pearls (What They Love to Ask)
- Vitamin C increases iron absorption (Fe³⁺ → Fe²⁺).
- Scurvy can coexist with iron deficiency, worsening fatigue.
- Smoking increases vitamin C requirements (oxidative stress).
- If the stem includes poor wound healing + bleeding + perifollicular hemorrhage, think scurvy first, not platelet/coag disorders.
- Collagen synthesis steps frequently appear:
- Vitamin C: hydroxylation
- Copper: cross-linking
- Zinc: matrix metalloproteinases/wound healing (broader association)
Quick Self-Check (1-Minute)
If you see:
- Bleeding + corkscrew hair/perifollicular hemorrhage → Vitamin C
- Bleeding + ↑ PT/INR → Vitamin K
- Neurologic + macrocytosis → B12
- Confusion/ataxia/ophthalmoplegia (alcohol use) → B1
- Kinky hair + neuro + connective tissue → Copper (Menkes)