Copper is one of those “small” trace elements that shows up everywhere on Step—usually disguised inside a vague anemia/neuro vignette or hiding behind a “bariatric surgery” stem. The trick is that copper deficiency doesn’t just cause one symptom; it knocks out multiple copper-dependent enzymes, so you get a pattern. This post breaks down a classic Q-bank-style vignette and then walks through why every answer choice matters—so you don’t fall for the distractors.
Tag: Biochemistry > Vitamins & Cofactors
The Clinical Vignette (Q-bank style)
A 52-year-old man presents with progressive gait instability and numbness in his feet for 6 months. He also reports fatigue. He had a Roux-en-Y gastric bypass 2 years ago. Exam shows decreased vibration and proprioception in the lower extremities with a wide-based gait. Labs show:
- Hemoglobin: 9.8 g/dL
- MCV: 104 fL
- WBC: 2,800/µL
- Platelets: 160,000/µL
Which of the following enzyme deficiencies best explains this patient’s condition?
A. Pyruvate dehydrogenase
B. Ceruloplasmin (ferroxidase)
C. Dihydrofolate reductase
D. Glutathione peroxidase
E. Phenylalanine hydroxylase
Step-wise Approach: What’s the Diagnosis?
This is copper deficiency until proven otherwise.
Why?
Key clues:
- History: Roux-en-Y gastric bypass → malabsorption risk (copper is absorbed mainly in the stomach and proximal small intestine)
- Neuro: sensory ataxia + loss of vibration/proprioception → posterior column dysfunction
- Heme/immune: anemia + leukopenia (often neutropenia) → copper deficiency classically causes anemia and leukopenia
High-yield association: Copper deficiency can mimic vitamin B12 deficiency neurologically (posterior column signs), and it can also cause cytopenias.
Correct Answer: B. Ceruloplasmin (ferroxidase)
What ceruloplasmin does
Ceruloplasmin is a copper-carrying protein made in the liver that also has ferroxidase activity:
- Converts Fe → Fe, allowing iron to bind transferrin for transport
- Without it (or without copper to support it), iron gets “stuck” and can’t be mobilized appropriately → anemia pattern consistent with impaired iron handling
Why copper deficiency causes anemia
Copper is required for normal iron metabolism via:
- Ceruloplasmin (ferroxidase): mobilizes iron into circulation
- Hephaestin (another copper-dependent ferroxidase in enterocytes): helps export iron from enterocytes to plasma
Net effect: impaired iron utilization/transport → anemia (can be normocytic or macrocytic in real life; stems often give macrocytosis because the neuro findings push you toward B12, but the leukopenia points you back to copper).
Why copper deficiency causes neurologic symptoms
Mechanism is not always tested as a single enzyme name, but it’s tied to disrupted function of copper-dependent enzymes involved in CNS integrity (classically myelopathy/peripheral neuropathy). On exams, think:
- Posterior column dysfunction, sensory ataxia
- Can mimic subacute combined degeneration (B12 deficiency)
Copper: The High-Yield Cheat Sheet
Major copper-dependent enzymes (testable)
| Enzyme | Requires | Key function | What deficiency looks like |
|---|---|---|---|
| Ceruloplasmin | Copper | Fe → Fe for transferrin loading | Anemia due to impaired iron mobilization |
| Cytochrome c oxidase (Complex IV) | Copper (and heme) | Oxidative phosphorylation, electron transfer | ↓ ATP production (nonspecific weakness), high-yield association in some stems |
| Dopamine β-hydroxylase | Copper | Dopamine → norepinephrine | Autonomic-type symptoms possible (rarely emphasized) |
| Lysyl oxidase | Copper | Cross-links collagen and elastin | Connective tissue weakness (more classic in Menkes) |
| Tyrosinase | Copper | Melanin synthesis | Hypopigmentation (more classic in Menkes) |
Classic causes of copper deficiency (Step-friendly)
- Bariatric surgery (Roux-en-Y)
- Malabsorption (celiac, chronic diarrhea)
- Excess zinc supplementation (zinc induces metallothionein in enterocytes → binds copper → lost in stool when enterocytes slough)
Classic findings
- Anemia (often with iron-handling pattern)
- Leukopenia/neutropenia
- Myelopathy/peripheral neuropathy (posterior column signs; can mimic B12 deficiency)
Now Destroy the Distractors (Why Every Answer Choice Matters)
A. Pyruvate dehydrogenase
Why it’s tempting: “Neuro symptoms + fatigue” can bait you into energy metabolism disorders.
Why it’s wrong: PDH deficiency is a congenital disorder (often infancy/childhood) with:
- Lactic acidosis
- Neurologic deficits (developmental delay)
- Sometimes associated with thiamine (B1) as a cofactor (TPP)
High-yield rule: PDH is about lactate and early-life presentation, not acquired post-bariatric cytopenias + posterior column signs.
C. Dihydrofolate reductase (DHFR)
Why it’s tempting: Macrocytosis with anemia → think folate pathway.
Why it’s wrong: DHFR is inhibited by methotrexate (and trimethoprim affects bacterial DHFR). DHFR-related issues cause megaloblastic anemia but don’t explain posterior column symptoms (that’s B12 more than folate) and don’t classically cause leukopenia in isolation unless severe drug effect/toxicity.
How to separate from copper deficiency:
- Folate deficiency: megaloblastic anemia, ↑ homocysteine, no neuro deficits
- Copper deficiency: anemia + leukopenia + neuro deficits (posterior column-like)
D. Glutathione peroxidase
What it tests: Selenium dependence.
Why it’s tempting: Trace element question + anemia-ish symptoms.
Why it’s wrong: Selenium deficiency is classically linked to:
- Keshan disease (cardiomyopathy)
- Sometimes impaired immune function or thyroid metabolism (deiodinases are selenium-dependent)
Not the best fit: Selenium deficiency doesn’t classically produce the anemia + leukopenia + posterior column dysfunction combo seen here.
High-yield association: If the stem screams cardiomyopathy in a geographic/nutrition context → selenium.
E. Phenylalanine hydroxylase
What it tests: PKU (BH4 cofactor).
Why it’s tempting: “Cofactor enzyme” vibe.
Why it’s wrong: PKU presents in infants/children, not as new neurologic symptoms post-gastric bypass. Also, PKU is characterized by:
- Intellectual disability (if untreated)
- Seizures
- “Musty” odor
- Hypopigmentation (due to less tyrosine → melanin)
High-yield tie-in: Phenylalanine hydroxylase requires BH4; deficiency → PKU. Completely different clinical universe than copper deficiency.
High-Yield Copper vs B12: The Common Step Trap
Both can cause posterior column findings. The differentiator is the blood count pattern and risk factors.
| Feature | Copper deficiency | Vitamin B12 deficiency |
|---|---|---|
| Neuro findings | Posterior column dysfunction, sensory ataxia | Posterior column + corticospinal (subacute combined degeneration) |
| CBC | Anemia + leukopenia/neutropenia common | Megaloblastic anemia; pancytopenia can occur but classically anemia |
| Risk factors | Bariatric surgery, excess zinc, malabsorption | Pernicious anemia, vegan diet, ileal disease/resection |
| Key biochem clues | Copper-dependent ferroxidase issues | ↑ Methylmalonic acid + ↑ homocysteine |
If a stem includes bariatric surgery + leukopenia, copper deficiency should jump way up your list.
Quick USMLE Takeaways (What to memorize)
- Copper deficiency classically causes:
- Anemia
- Leukopenia (neutropenia)
- Posterior column-like neurologic deficits
- Copper is required for ceruloplasmin ferroxidase activity → iron mobilization (Fe → Fe for transferrin).
- Common causes: Roux-en-Y, malabsorption, excess zinc.
- Big trap: can mimic B12 deficiency neurologically—look for leukopenia and the surgical/supplement history.