Ans) a (USG abdomen) Ref Nelson 18th ed pi555Above clinical scenario is about the classical presentation of hypertrophic pyloric stenosis.Ultrasound examination confirms the diagnosis in the majority of cases and allots an earlier diagnosis in infants with suspected disease but no pyloric mass on physical examination.Criteria for diagnosis include pyloric thickness >4 mm or an overall pyloric length >14 mm Ultrasonography has a sensitivity of ?95%When contrast studies are performed, they demonstrate an elon-gated pyloric channel, a bulge of the pyloric muscle into the antrum (shoul-der sign), and parallel streaks of barium seen in the narrowed channel, producing a "double tract sign"Hypertrophic pyloric stenosisHypertrophic pyloric stenosis occurs in 1 -3/1,000 infants in the United States. Males, (especially first-borns) are affected approximately four times as often as females. The offspring of a mother and. to a lesser extent, the father who had pyloric stenosis are at higher risk for pyloric stenosis. Pyloric stenosis develops in approximately 20% of the male and 10% of the female descendants of a mother who had pyloric stenosis. The inci-dence of pyloric stenosis is increased in infants with type B and O blood groups. Pyloric stenosis is associated with other congenital defects, in-cluding tracheoesophageal fistula and hypoplasia or agenesis of the infe-rior labial frenulum.ETIOLOGY.The cause of pyloric stenosis is unknown, but many factors have been implicated. Pyloric stenosis is usually not present at birth and is more concordant in monozygotic than dizygotic twins. Pyloric stenosis has been associated with eosinophilic gastroenteritis, Apert syndrome, Zellweger syndrome, trisomy 18, Smith-Lemli-Opitz syndrome, and Cornelia de Lange syndrome. A variable association has been found with the use of erythromycin in neonates when administered for pertussis postexposure prophy-laxis. Reduced levels of pyloric nitric oxide synthase have been found with altered expression of the neuronal nitric oxide synthase (nNOS) exon lc regulatory region, which influences the expression of the nNOS gene. Reduced nitric oxide may contribute to the pathogenesis of pyloric stenosisCLINICAL MANIFESTATIONSNonbilious vomiting is the initial symptom of pyloric stenosis. The vomiting may or may not be projectile initially but is usually progressive, occurring immediately after a feeding. The vomiting usually starts after 3 wk of age, but symptoms may develop as early as the 1 st wk of life and as late as the 5th mo. As vomiting continues, a progressive loss of fluid, hydrogen ion, and chloride leads to hypochloremic metabolic alkalosis. Serum potassium levels are usually maintained, but there may be a total body potassium deficit. Jaundice associated with a decreased level of glucuronyl trans-ferase is seen in ?5% of affected infants. The indirect hyperbiliru- binemia usually resolves promptly after relief of the obstruction.The diagnosis has traditionally been established by palpating the pyloric mass. The mass is firm, movable, ?2 cm in length, olive shaped, hard, best palpated from the left side, and located above and to the right of the umbilicus in the mid epigastrium beneath the liver edge. In healthy infants, feeding can be an aid to the diagnosis. After feeding, there may be a visible gastric peristaltic wave that progresses across the abdomen.