Q-Bank Breakdown: Ehlers-Danlos Syndrome — Why Every Answer Choice Matters
Tag: Biochemistry > Collagen & Connective Tissue
Collagen questions are classic USMLE territory because they blend biochemistry, cell biology, and clinical pattern recognition. In this Q-bank breakdown, we’ll walk through a high-yield Ehlers-Danlos syndrome (EDS) vignette, nail the correct answer, and then dismantle every distractor—because on test day, knowing why something is wrong is often what gets you the point.
The Vignette (USMLE-Style)
A 19-year-old woman presents with long-standing joint “looseness” and frequent sprains. She reports easy bruising and notes that small cuts “split open” and heal with wide, thin scars. On exam, she can hyperextend her fingers and elbows well beyond normal range, and her skin is soft and hyperextensible. Family history reveals similar findings in her father.
Question: The underlying defect most likely involves impaired synthesis or structure of which of the following?
Answer Choices
A. Decreased activity of lysyl hydroxylase
B. Defective type III collagen
C. Decreased activity of prolyl hydroxylase due to vitamin C deficiency
D. Decreased activity of homogentisate oxidase
E. Defective fibrillin-1
Step 1: Identify the Syndrome (Pattern Recognition)
This vignette screams Ehlers-Danlos syndrome (EDS):
- Hyperextensible skin
- Hypermobile joints
- Easy bruising
- Poor wound healing → wide/atrophic (“cigarette paper”) scars
- Often autosomal dominant (many types)
✅ The best match here is B. Defective type III collagen (classically associated with the vascular type of EDS, and a common distractor pairing against Marfan).
Correct Answer Deep Dive: B. Defective Type III Collagen
Why type III collagen?
Type III collagen is a major component of:
- Blood vessels (especially large arteries)
- Uterus
- Bowel wall
- Skin (with type I)
So defects can produce:
- Easy bruising
- Fragile tissues
- Risk of arterial rupture, intestinal perforation, and uterine rupture (especially in pregnancy)
High-yield association
- EDS, vascular type → COL3A1 mutation → abnormal type III collagen
- Presents with thin, translucent skin, easy bruising, and potentially catastrophic vascular/organ rupture
USMLE tip: If the vignette mentions arterial dissection/rupture, bowel perforation, or uterine rupture, think vascular EDS (type III collagen).
Collagen Biochemistry Refresher (Just Enough for USMLE)
Key synthesis steps (high yield)
- Translation in RER → preprocollagen
- Hydroxylation of proline/lysine in RER
- Requires vitamin C, Fe²⁺, and O₂
- Glycosylation and triple helix formation → procollagen
- Secretion
- Cleavage of N- and C-terminal propeptides → tropocollagen
- Cross-linking (lysine residues) via lysyl oxidase (requires copper)
Now Kill the Distractors (Why Every Answer Choice Matters)
A. Decreased activity of lysyl hydroxylase
This is also Ehlers-Danlos, but it’s a different mechanism.
- Lysyl hydroxylase hydroxylates lysine residues (RER step)
- Leads to impaired collagen crosslinking stability
- Classic association: EDS (some types), often described with:
- Hypermobile joints
- Hyperextensible skin
- Easy bruising
Why it’s wrong here:
The question is set up to target a structural collagen type defect rather than a hydroxylation enzyme defect. On many USMLE items, the vascular catastrophe clue pushes you to type III collagen. If the stem emphasized “impaired hydroxylation” or “decreased hydroxylysine,” A would climb.
Test-taking pearl:
- EDS can be enzyme-related or collagen-type-related. Read the stem for what it’s testing: biochemical step vs collagen subtype.
C. Decreased activity of prolyl hydroxylase due to vitamin C deficiency
This describes scurvy, not EDS.
Scurvy key features:
- Bleeding gums
- Petechiae, perifollicular hemorrhage
- Poor wound healing
- Corkscrew hairs
- Anemia (often)
- Due to impaired hydroxylation → unstable triple helix → weak collagen
Why it’s wrong here:
EDS is typically genetic and presents with hypermobility and skin hyperextensibility. Scurvy is nutritional and more hemorrhagic/systemic with classic mucosal findings.
USMLE hook:
- Vitamin C is required for prolyl and lysyl hydroxylation in the RER.
D. Decreased activity of homogentisate oxidase
This is alkaptonuria.
Alkaptonuria key features:
- Dark urine (turns dark on standing)
- Ochronosis (bluish-black pigmentation of connective tissues)
- Arthralgias/early degenerative joint disease
- Due to tyrosine degradation defect (homogentisic acid accumulation)
Why it’s wrong here:
No pigmentation changes, dark urine, or early severe arthritis pattern consistent with ochronosis. Also, this is amino acid metabolism, not collagen processing.
E. Defective fibrillin-1
This is Marfan syndrome.
Marfan key features:
- Tall stature, long limbs (arachnodactyly)
- Lens subluxation downward/outward
- Aortic root dilation → aneurysm/dissection
- Mutation in FBN1 → impaired microfibrils → altered TGF-β signaling
Why it’s wrong here:
Marfan is a fibrillin disorder, not a collagen synthesis disorder, and the vignette focuses on:
- Hyperextensible skin
- Easy bruising
- Poor wound healing with abnormal scarring
These are more EDS-coded than Marfan-coded.
High-yield differentiation: EDS vs Marfan
- EDS: skin hyperextensibility + atrophic scars + easy bruising
- Marfan: lens + aorta + tall habitus; skin findings are not the centerpiece
High-Yield Collagen Types (You Should Memorize)
- Type I: bone, skin, tendon, ligaments, dentin, fascia, cornea
- Disease: osteogenesis imperfecta, some EDS
- Type II: cartilage, vitreous body, nucleus pulposus
- Disease: achondroplasia (related to cartilage), Stickler syndrome (commonly tested in Step 2 contexts)
- Type III: reticular fibers, blood vessels, uterus, bowel wall
- Disease: vascular EDS
- Type IV: basement membranes (glomeruli, lens capsule, cochlea)
- Disease: Alport, Goodpasture (autoimmune target)
Rapid-Fire USMLE Takeaways
- EDS core triad: hypermobile joints + hyperextensible skin + tissue fragility
- Vascular EDS = type III collagen defect → arterial/uterine/bowel rupture risk
- Scurvy: ↓ hydroxylation (vitamin C) → bleeding gums, corkscrew hairs
- Menkes: copper transport defect → ↓ lysyl oxidase → weak collagen (kinky hair, neuro issues)
- Marfan: fibrillin-1 defect (not collagen) → lens + aorta + tall habitus
Quick Self-Check (1-Minute)
If a question asks:
- “Which collagen is in blood vessels and reticular fibers?” → Type III
- “Which step requires vitamin C?” → Prolyl/lysyl hydroxylation in RER
- “Which enzyme is copper-dependent for crosslinking?” → Lysyl oxidase
- “Basement membrane collagen?” → Type IV