## Why menstrual iron loss is the correct answer Hereditary hemochromatosis type 1 (HFE-related) exemplifies **sex-influenced inheritance**: an autosomal recessive trait where both sexes inherit the mutation equally, but males manifest disease 5–10 times more frequently and earlier (typically 40–60 years) than homozygous females (typically 50–70 years, post-menopausal). The biological basis is that women lose approximately 30 mg of iron per menstrual cycle and ~700 mg per pregnancy, which delays iron accumulation in tissues. Although both siblings carry identical C282Y homozygosity, the brother's iron stores have accumulated unchecked since puberty, whereas the sister's menstrual losses have provided continuous iron depletion, keeping her ferritin and tissue iron burden below the clinical threshold. Penetrance of C282Y homozygosity is 28% in males but only 1% in females for clinical disease—a difference entirely attributable to menstrual and reproductive iron loss (Harrison's Principles of Internal Medicine, 21st ed., Ch. 414; EASL 2022 Guidelines). ## Why each distractor is wrong - **X-linked inheritance**: HFE is located on chromosome 6p22.2 (autosomal), not the X chromosome. X-linked recessive inheritance is represented by pattern **B** in the diagram, not **A**. Skewed X-inactivation does not apply to autosomal genes. - **Estrogen-mediated hepcidin upregulation**: While estrogen does have some modulatory effects on iron metabolism, the primary protective mechanism in women with hemochromatosis is iron loss through menstruation and pregnancy, not increased hepcidin production. The HFE mutation itself impairs hepcidin upregulation; estrogen does not compensate for this defect. - **DMT1 downregulation by sex hormones**: There is no established mechanism by which sex hormones downregulate DMT1 (divalent metal transporter 1) in females with HFE mutations. The pathophysiology of HFE hemochromatosis centers on loss of HFE-mediated hepcidin upregulation, leading to unopposed ferroportin activity—not on primary DMT1 regulation by sex hormones. **High-Yield:** Sex-influenced inheritance = autosomal recessive trait with male predominance and earlier/more severe disease due to lack of protective iron loss mechanisms (menstruation, pregnancy) in females. [cite: Harrison's Principles of Internal Medicine, 21st ed., Ch. 414; EASL Clinical Practice Guidelines on HFE Hemochromatosis 2022; Thompson & Thompson Genetics in Medicine, 8th ed.]
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