Correct Answer: A. Intussusception
Intussusception is the most common cause of acute abdominal obstruction in infants and toddlers (6 months to 3 years), with peak incidence at 9–12 months. The classic clinical triad is colicky abdominal pain, a palpable right lumbar mass (the "sausage-shaped" intussusceptum), and bloody mucoid stools ("currant jelly" stools). The barium enema findings are pathognomonic: the "target sign" (concentric rings on axial view) or "coiled spring" appearance (on longitudinal view) represents the invaginated bowel segment. In India, most cases are idiopathic (viral-triggered lymphoid hyperplasia in Peyer's patches), though lead points (Meckel's diverticulum, polyps, lymphoma) must be excluded in children >3 years or recurrent cases. The barium enema is both diagnostic and therapeutic—hydrostatic or pneumatic reduction succeeds in 60–90% of uncomplicated cases. The right lower quadrant location (ileocolic intussusception) accounts for ~90% of cases. Absence of peritonitis signs and the acute presentation in an otherwise healthy infant strongly support this diagnosis over other causes of obstruction.
Why the other options are wrong
B. Intestinal obstruction — While intussusception is a form of mechanical obstruction, this option is too nonspecific and lacks the discriminating clinical and radiological features. 'Intestinal obstruction' does not capture the characteristic right lumbar mass, the age group (infants/toddlers), or the pathognomonic barium enema signs (target/coiled spring). NBE uses this as a trap for students who recognize obstruction but fail to narrow the differential to the specific diagnosis. C. Duodenal atresia — Duodenal atresia presents in the neonatal period (first 24–48 hours of life) with bilious vomiting and abdominal distension, not in an older infant with a palpable mass. The classic radiological sign is the 'double bubble' (dilated stomach and proximal duodenum) on plain X-ray, not a barium enema finding. This option traps students who confuse congenital obstructive lesions with acquired intussusception. D. Volvulus — Volvulus (midgut or sigmoid) presents with acute, severe obstruction and rapid deterioration with signs of peritonitis and shock if ischemia occurs. Unlike intussusception, volvulus does not produce a palpable abdominal mass in the right lumbar region. Barium enema in volvulus shows a 'bird's beak' or 'whirlpool' sign, not the target/coiled spring appearance. Volvulus is more common in older children and adults with predisposing factors (malrotation, adhesions).
High-Yield Facts
- Peak age of intussusception: 6 months to 3 years, with maximum incidence at 9–12 months in Indian populations.
- Classic triad: colicky abdominal pain + right lumbar mass + currant jelly stools—any two should trigger intussusception suspicion.
- Barium enema signs: 'target sign' (axial) or 'coiled spring' (longitudinal)—both diagnostic and therapeutic (60–90% reduction success).
- Ileocolic intussusception (~90% of cases) presents with right lower quadrant mass; jejunoileal and colocolic types are less common.
- Idiopathic intussusception (viral lymphoid hyperplasia) is the rule in children <3 years; lead points (Meckel's, polyps, lymphoma) must be excluded in older children or recurrent cases.
- Ultrasound (target/doughnut sign) is the first-line imaging in modern practice; barium enema is reserved for therapeutic reduction when ultrasound confirms diagnosis.
Mnemonics
MASS in Intussusception Mass (right lumbar) + Abdominal pain (colicky) + Stools (currant jelly) + Summer/Spring (peak 6–36 months). Use this to recall the clinical triad and age group instantly. Barium Signs: TARGET or SPRING TARGET sign = axial view (concentric rings); SPRING = longitudinal view (coiled appearance). Both are pathognomonic for intussusception on barium enema.
NBE Trap
NBE pairs "intestinal obstruction" as a distractor to test whether students can narrow the differential from a generic obstruction to the specific diagnosis using clinical context (age, mass location, barium findings). Students who recognize obstruction but lack knowledge of intussusception's pathognomonic features may incorrectly choose the broader option.
Clinical Pearl
In Indian pediatric practice, a toddler presenting with acute colicky pain and a right lumbar mass should trigger immediate ultrasound (target sign) or barium enema—early diagnosis and hydrostatic reduction within 24–48 hours prevents ischemia, perforation, and the need for surgery. Delayed recognition is a common cause of morbidity in resource-limited settings.
_Reference: OP Ghai Pediatrics Ch. 12 (Gastroenterology); Harrison Ch. 297 (Pediatric Emergencies)_