Correct Answer: A. IgA tissue transglutaminase antibody
This clinical presentation—fatigue, chronic diarrhoea, weight loss, bone pain, and osteoporosis in a young girl—is pathognomonic for coeliac disease (CD). The constellation of iron deficiency anaemia (from malabsorption of iron in the proximal small intestine) and osteoporosis (from calcium and vitamin D malabsorption) is the discriminating feature that points directly to a malabsorption disorder affecting the small bowel. In coeliac disease, gluten exposure triggers an autoimmune response in genetically predisposed individuals (HLA-DQ2/DQ8 positive), leading to villous atrophy and loss of absorptive surface. IgA tissue transglutaminase (tTG) antibody is the gold-standard screening and diagnostic test for coeliac disease in India and globally. It has >95% sensitivity and specificity in untreated disease. The IgA isotype is chosen because IgA deficiency occurs in ~3% of the population; if IgA-deficient, total IgG anti-tTG is measured instead. This test is far superior to endoscopic biopsy for initial screening in primary care and outpatient settings. Early diagnosis prevents long-term complications including secondary osteoporosis, infertility, and increased malignancy risk. In the Indian context, coeliac disease is increasingly recognized in children presenting with "failure to thrive" and chronic diarrhoea, making serological screening essential before invasive investigation.
Why the other options are wrong
B. C-peptide levels — C-peptide is a marker of endogenous insulin secretion, used to evaluate diabetes mellitus or insulinoma. While type 1 diabetes can coexist with coeliac disease (both autoimmune), the clinical presentation here—diarrhoea, bone pain, osteoporosis—is not suggestive of diabetes. There is no mention of polyuria, polydipsia, or hyperglycaemia. This is a distractor for students who recognize autoimmune disease but confuse the organ system involved. C. TSH levels — TSH is used to screen for hypothyroidism, which can cause fatigue and weight gain (not weight loss). Autoimmune thyroiditis can coexist with coeliac disease, but the clinical picture—chronic diarrhoea, bone pain, osteoporosis—is not typical of thyroid disease alone. Thyroid dysfunction does not cause malabsorption or secondary osteoporosis. This option exploits the fact that both are autoimmune conditions but misses the GI malabsorption clue. D. Urine sugar and ketone — Urine sugar and ketone testing screens for diabetes mellitus or diabetic ketoacidosis. There is no clinical evidence of diabetes in this case—no polyuria, polydipsia, or acute metabolic derangement. Glycosuria and ketonuria are not features of coeliac disease. This is a trap for students who see 'weight loss' and 'fatigue' and incorrectly assume metabolic disease rather than recognizing the malabsorption pattern.
High-Yield Facts
- IgA tissue transglutaminase (tTG) antibody is the single best screening test for coeliac disease with >95% sensitivity and specificity in untreated disease.
- Coeliac disease presents with iron deficiency anaemia, osteoporosis, and chronic diarrhoea due to villous atrophy and malabsorption in the small intestine.
- HLA-DQ2 and HLA-DQ8 are present in >95% of coeliac disease patients; negative serology makes CD diagnosis unlikely.
- IgA deficiency occurs in ~3% of the population; if IgA-deficient, total IgG anti-tTG antibody should be measured to avoid false-negative results.
- Coeliac disease screening is recommended in all children with chronic diarrhoea, failure to thrive, iron deficiency anaemia, or osteoporosis in India.
Mnemonics
CD Malabsorption Triad Iron (anaemia) + Calcium (osteoporosis) + Fat-soluble vitamins (diarrhoea, weight loss) = Coeliac Disease. Remember: small bowel villous atrophy → loss of absorptive surface → multiple micronutrient deficiencies. tTG Test Timing Test before gluten withdrawal; antibodies disappear within weeks of a gluten-free diet. In India, serology is done first; biopsy (endoscopy + duodenal biopsy) confirms diagnosis if serology is positive and clinical suspicion is high.
NBE Trap
NBE pairs "autoimmune disease" with multiple organ systems (thyroid, pancreas, kidney) to lure students away from the GI malabsorption clue. The key discriminator is the combination of iron deficiency + osteoporosis + chronic diarrhoea, which is pathognomonic for small-bowel malabsorption, not endocrine or metabolic disease.
Clinical Pearl
In Indian paediatric practice, coeliac disease is often missed because it presents as "chronic diarrhoea" or "failure to thrive"—conditions attributed to infectious causes or malnutrition. A single IgA-tTG test in primary care can identify CD early, prevent growth stunting, and avoid unnecessary endoscopy in seronegative cases. Early diagnosis is critical in adolescent girls to prevent secondary osteoporosis and infertility complications.
_Reference: Harrison Ch. 297 (Coeliac Disease); Robbins Ch. 17 (Small Intestine); Park's Textbook of Preventive and Social Medicine (Nutritional Deficiency Disorders)_