Which investigation is best for a child presenting with abdominal pain and anemia, showing basophilic stippling in the blood film?

In the case of a child presenting with abdominal pain and anemia, the presence of basophilic stippling in the blood film is a key indicator pointing towards lead poisoning. Basophilic stippling occurs due to the aggregation of ribosomal RNA in red blood cells, which can happen in conditions associated with lead toxicity.

Lead levels are the most relevant investigation in this scenario because they directly assess the presence of lead in the child's system, which can lead to both abdominal symptoms and anemia. Lead poisoning can manifest with neurological symptoms, gastrointestinal issues, and hematological abnormalities, including anemia and basophilic stippling.

While a complete blood count provides valuable information regarding the child's overall blood parameters, it does not specifically identify the cause of the anemia or the stippling. Iron studies are primarily used to assess iron deficiency anemia and would not provide insight regarding lead poisoning. Similarly, a reticulocyte count would indicate bone marrow response to anemia but would not pinpoint lead as the underlying issue.

Therefore, measuring lead levels is the most appropriate and direct investigation to diagnose lead poisoning in this clinical context.