You are evaluating a small for gestational age neonate; you find the following features: hypertonia, prominent occiput, short sternum, and clenching of fists.

Correct Answer: C

Rationale: Edward syndrome (trisomy 18, C) features hypertonia, clenched fists, and prominent occiput. Rationale: Down (A) has hypotonia; Patau (B) has polydactyly.

Correct Answer: C

Rationale: Translocation Down syndrome (4% of cases) involves chromosome 21 attaching to 13, 14, 15, or 21. Carriers are phenotypically normal, but recurrence risk is high (10-15% for mothers, lower for fathers). Phenotypes are not milder than trisomy 21.

Correct Answer: C

Rationale: Karyotyping detects large chromosomal changes (deletions, inversions) but not small nucleotide insertions or point mutations, which require molecular techniques like sequencing.

Correct Answer: A

Rationale: Deletion of a 30 Mb region removes many genes, likely causing severe disease (e.g., developmental delay). Duplications or inversions (pericentric/paracentric) may disrupt gene dosage or structure but are typically less severe than outright loss.

Correct Answer: A

Rationale: No family history could result from a de novo mutation (new in the patient), germline mosaicism (parental gonadal mutation), anticipation (e.g., trinucleotide expansion), or reduced penetrance (hidden in prior generations). All are plausible.