What is the most common underlying cause of anemia in a child presenting with developmental delays and a peripheral smear showing basophilic stippling?

The presence of developmental delays in a child, combined with a peripheral smear showing basophilic stippling, strongly suggests lead poisoning as the underlying cause of anemia. Basophilic stippling is a hallmark feature seen in the blood smear of individuals exposed to lead, which results from the inhibition of enzymes involved in heme synthesis.

In children, lead exposure can lead to varying degrees of toxicity, affecting nervous system development and resulting in developmental delays. The anemia caused by lead poisoning is due to both hemolysis and impaired hemoglobin synthesis, which can further manifest as microcytic anemia.

In this context, lead is well-known for its neurotoxic effects, particularly in young children whose developing systems are more vulnerable. The correlation between lead toxicity and basophilic stippling strengthens the case for lead poisoning being the most common underlying cause of anemia in this patient scenario.