Step 1 topics → Biochemistry → Minerals (Zinc, Copper, Iron, Selenium, Iodine)

Minerals (Zinc, Copper, Iron, Selenium, Iodine)

USMLE Step 1 trap: Confuses zinc deficiency features with iron deficiency anemia. Zinc deficiency causes impaired wound healing, hypogonadism, anosmia, dysgeusia, and dermatitis — not anemia.

Minerals like zinc, copper, selenium, and iodine show up on USMLE Step 1 mostly through clinical vignettes — a patient with poor wound healing, a child with kinky hair and neurodegeneration, or someone from a selenium-deficient region with new-onset cardiomyopathy. Students consistently conflate Wilson disease and Menkes disease — both involve copper transport ATPases, but Wilson causes copper overload while Menkes causes functional copper deficiency — and mix up copper-dependent and zinc-dependent enzyme lists. The exam isn't testing whether you memorized a list; it's testing whether you can link the mineral to its biochemical role and then to the clinical phenotype.

The trickiest part of this topic is keeping the copper diseases straight. Wilson disease (ATP7B) and Menkes disease (ATP7A) both involve copper transport, but they are clinical opposites — Wilson causes copper overload, Menkes causes functional copper deficiency. Students routinely conflate them because both are X-linked or autosomal with 'copper' in the story. The enzyme lists also trip people up: copper-dependent enzymes (ceruloplasmin, lysyl oxidase, dopamine β-hydroxylase, cytochrome c oxidase, superoxide dismutase) are completely different from zinc-dependent enzymes (carbonic anhydrase, alcohol dehydrogenase, collagenase).

USMLE Step 1 also tests selenium and iodine, though less frequently. Selenium's key hook is Keshan disease — a dilated cardiomyopathy from selenium deficiency, caused by loss of glutathione peroxidase activity. Iodine shows up via thyroid synthesis and goiter physiology. Don't skip these just because they're lower yield; a single vignette about a child in a rural mountainous region or a patient on long-term TPN can make selenium or iodine the answer.

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Common misconceptions

Common mistake
Wrong: Zinc deficiency primarily causes anemia because zinc is a metal like iron.
Right: Zinc deficiency causes impaired wound healing, hypogonadism, anosmia, dysgeusia, and dermatitis — not anemia.
Zinc and iron are both metals, but their biochemical roles are entirely different — zinc is a structural and catalytic cofactor in enzymes involved in DNA synthesis, cell proliferation, and wound repair, not in hemoglobin or red cell production. Zinc deficiency produces a constellation of findings: impaired wound healing, hypogonadism, anosmia (loss of smell), dysgeusia (altered taste), and acrodermatitis enteropathica (perioral and acral dermatitis). If you see anemia in a mineral deficiency question, think iron (microcytic), B12/folate (macrocytic), or copper (sideroblastic-like) — not zinc.
Common mistake
Wrong: Copper-dependent enzymes are the same as zinc-dependent enzymes (e.g., carbonic anhydrase, alcohol dehydrogenase).
Right: Copper-dependent enzymes include ceruloplasmin, lysyl oxidase, dopamine β-hydroxylase, cytochrome c oxidase, and superoxide dismutase.
Zinc-dependent and copper-dependent enzymes are frequently conflated because both are transition metals that act as enzyme cofactors, but the lists don't overlap. Copper-dependent enzymes have a useful mnemonic: think of the major ones as ceruloplasmin (ferroxidase), lysyl oxidase (crosslinks collagen and elastin), dopamine β-hydroxylase (makes norepinephrine), cytochrome c oxidase (electron transport), and superoxide dismutase (antioxidant defense). Zinc-dependent enzymes include carbonic anhydrase, alcohol dehydrogenase, and matrix metalloproteinases — different functions, different clinical consequences when deficient.
Common mistake
Wrong: Both Wilson disease and Menkes disease result in copper accumulation in tissues.
Right: Wilson disease causes copper accumulation (ATP7B defect), while Menkes disease causes copper deficiency in tissues due to impaired intestinal export (ATP7A defect).
Both diseases involve the same family of copper-transporting ATPases, which makes them easy to confuse, but the functional consequences are opposite. In Wilson disease, ATP7B is defective — copper can't be exported into bile or incorporated into ceruloplasmin, so it accumulates in the liver, basal ganglia, and Descemet membrane (Kayser-Fleischer rings). In Menkes disease, ATP7A is defective — copper absorbs into intestinal enterocytes fine but can't be exported into the bloodstream, so the body is functionally copper-deficient despite ingesting copper. The result is loss of all copper-dependent enzyme activity: kinky/brittle hair (loss of lysyl oxidase crosslinking), neurodegeneration, and connective tissue fragility.
Common mistake
Gap: Missing the link between selenium, glutathione peroxidase, and Keshan disease
Selenium is a component of glutathione peroxidase, which reduces hydrogen peroxide and protects cells from oxidative damage; deficiency causes Keshan disease (dilated cardiomyopathy).
Selenium is a micronutrient that gets incorporated into selenoproteins, the most important of which for USMLE Step 1 is glutathione peroxidase. This enzyme neutralizes hydrogen peroxide and lipid peroxides, protecting cells from oxidative damage — it's the selenium-dependent counterpart to catalase and superoxide dismutase. When selenium is deficient, glutathione peroxidase activity drops, oxidative stress accumulates preferentially in cardiac myocytes, and the result is Keshan disease: a dilated cardiomyopathy seen classically in selenium-poor regions of China or in patients on prolonged parenteral nutrition without selenium supplementation.
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What the exam tests

  1. Given a patient with poor wound healing, anosmia, dysgeusia, hypogonadism, or dermatitis around orifices, identify zinc deficiency as the cause — and distinguish it from iron deficiency, which causes anemia, not these features.
  2. Identify which enzymes depend on copper (ceruloplasmin, lysyl oxidase, dopamine β-hydroxylase, cytochrome c oxidase, superoxide dismutase) versus zinc, and predict what breaks down when copper is deficient.
  3. Distinguish Wilson disease (ATP7B mutation → copper accumulates in liver, brain, eye) from Menkes disease (ATP7A mutation → copper cannot exit intestinal cells → functional copper deficiency in tissues), including their clinical presentations and inheritance.
  4. Recognize Keshan disease (dilated cardiomyopathy) as the clinical consequence of selenium deficiency, mechanistically linked to loss of glutathione peroxidase and increased oxidative damage.

Can you avoid these mistakes?

A 6-month-old boy has sparse, kinky, hypopigmented hair, progressive neurodegeneration, and connective tissue laxity. Serum copper and ceruloplasmin are low. What is the defective protein, and why is serum copper low despite normal dietary intake?
A patient on long-term total parenteral nutrition (TPN) develops new-onset dilated cardiomyopathy. Which trace element deficiency explains this, what enzyme is lost, and what is the resulting biochemical consequence?
A 25-year-old man with liver cirrhosis and psychiatric symptoms is found to have Kayser-Fleischer rings on slit-lamp exam. His serum ceruloplasmin is low. Explain why ceruloplasmin is low in this disease even though copper is accumulating in tissues.
A vignette describes a patient with perioral and acral dermatitis, loss of smell, delayed wound healing after surgery, and hypogonadism. A classmate says this sounds like iron deficiency. What is the correct diagnosis, and how do you distinguish it from iron deficiency at the bedside and on labs?

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