Children with Down syndrome are most likely diagnosed clinically in the neonatal period. All the following are characteristic facial features EXCEPT

Correct Answer: B

Rationale: Down syndrome features include macroglossia (A), hypoplastic midface (C), flat nasal bridge (D), and down-slanting fissures (E); flattened occiput (B) is not typical. Rationale: Occiput is often normal or prominent.

Correct Answer: B

Rationale: Tay-Sachs is an autosomal recessive disorder caused by a single genetic defect in the HEXA gene, leading to enzyme deficiency. While more common in Ashkenazi Jews, it’s not exclusive to them. It affects both sexes, lacks genetic heterogeneity in its classic form, and is infantile-onset, not adult.

Correct Answer: C

Rationale: Multifactorial inheritance involves multiple genes and environment, with sex predilection (e.g., pyloric stenosis), recurrence rates of 2-10%, and twin concordance of 20-63%. Severe symptoms can increase risk. Symptomatic carriers apply to monogenic traits, not multifactorial.

Correct Answer: D

Rationale: The ‘double bubble’ sign on KUB indicates duodenal atresia, common in Down syndrome, causing a gas bubble in the stomach and proximal duodenum. Hirschsprung, meconium ileus, and plugs cause distal obstructions. Pyloric atresia is rarer and doesn’t produce this sign.

Correct Answer: D

Rationale: Aneuploidy (e.g., trisomy 21) is the most common chromosomal mutation in humans, especially in live births or conceptions. Indels and CNVs are frequent but less so, and polyploidy is rare in humans.