What is the best investigation for confirming a diagnosis in a child with diarrhoea, anaemia, and bulky, frothy stools?

In the context of a child with diarrhea, anemia, and bulky, frothy stools, confirming the diagnosis is crucial for appropriate management. The symptoms described are suggestive of conditions associated with malabsorption, one of the most common being celiac disease or other enteropathies.

A small bowel biopsy is considered the gold standard for diagnosing celiac disease. It allows for direct assessment of intestinal mucosal integrity, helping to identify changes such as villous atrophy, which are indicative of conditions that cause malabsorption. The characteristics of bulky, frothy stools often point to fat malabsorption (steatorrhea), further supporting the need for a histological examination of the small intestine.

While stool sample analysis can provide valuable insights, such as the presence of fat, inflammation, and pathogens, it does not definitively confirm a diagnosis as effectively as a biopsy. Testing blood electrolytes and conducting an abdominal ultrasound may help in evaluating the child's overall health or in ruling out other conditions, but they are not definitive investigations for the disorders indicated by the symptoms.

Thus, a small bowel biopsy provides the most reliable confirmation of the suspected diagnosis based on the symptom profile presented.