Breast-feeding is a very misleading answer. The reason infants can develop iron deficiency is because they are often only breastfed and there is low iron in breast milk. They are encouraged to be given food by the 6th month that contains iron instead of only being breastfed.
This answer makes it seem like breast-feeding causes iron deficiency when it doesn't
Source: https://next.amboss.com/us/article/pT0L72
see the section labeled "Risk factors for pediatric IDA"
Says
"Poor dietary intake of iron-fortified or iron-rich foods, e.g.:
Exclusively breastfed infants > 4 months of age not receiving iron prophylaxis
Early (i.e., < 12 months of age) and excessive (> 24 ounces/700 mL per day) intake of nonfortified cow's milk
Restrictive diets: malnutrition (mainly in resource-limited countries), eating disorders (including pica), picky eating"
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