The most likely diagnosis is celiac disease. Celiac disease is an autoimmune condition caused by a reaction to gluten ingestion. Symptoms include, but are not limited to, weight loss, suboptimal growth and delayed puberty, bloating, diarrhea or constipation. The diarrhea in celiac disease is non-bloody. The pattern of failure to thrive only after introduction of solid foods makes celiac disease the most likely diagnosis in this case. If celiac disease is suspected, anti-Tissue Transglutaminase Antibody test is the most appropriate initially screening test, and if positive, the patient should be referred to gastroenterology for endoscopy and biopsy, prior to recommending dietary changes. An IgA is included because anti-TTG is an IgA antibody and patients with IgA deficiency may have a falsely negative anti-TTG result. If failure to thrive is significant, CBC and albumin would be helpful to assess the patient’s nutritional status.