What is the most likely cause of anemia in a 5-year-old girl with a history of normal hemoglobin levels but now at 10.0 g/dL?

The most likely cause of anemia in a 5-year-old girl who previously had normal hemoglobin levels and now presents with a hemoglobin level of 10.0 g/dL is iron deficiency. In children, iron deficiency anemia is quite common due to several factors, including inadequate dietary intake of iron, increased iron requirements during periods of rapid growth, and possibly increased losses from minor injuries or infections.

At this age, children may have diets that are insufficient in iron-rich foods, especially if they have transitioned from infant formula or breast milk to a more varied diet without proper iron supplementation. Symptoms of iron deficiency may include fatigue, pallor, and irritability. In this age group, the implications of unrecognized or untreated iron deficiency anemia can also lead to developmental delays and cognitive impairment, making it crucial to identify and treat.

While vitamin B12 and folate deficiencies can also cause anemia, they are less common in young children compared to iron deficiency. Chronic disease can lead to anemia as well but is typically associated with other underlying health issues. Therefore, given the context of a previously healthy child now showing signs of anemia, iron deficiency is the most straightforward and likely explanation.