A full-term female newborn is examined shortly after birth. She appears to be small for gestational age, and she has excess skin on the nape of the neck and lymphedema of the hands and feet. Chromosomal analysis shows some cells with a normal 46,XY karyotype and some cells with a 45,X karyotype. Which of the following mechanisms best explains this cytogenetic abnormality?

Correct Answer: A

Rationale: Mosaicism (46,XY/45,X) arises from mitotic nondisjunction post-fertilization, losing an X or Y in some cells.

Correct Answer: C

Rationale: Amniotic fluid (C) is used in amniocentesis to study fetal chromosomes. Rationale: It contains fetal cells (e.g., amniocytes) that can be cultured for karyotyping, unlike lymphocytes (postnatal) or cheek swabs (not fetal). Skin biopsy and bone marrow are less common prenatally.

Correct Answer: B

Rationale: Giemsa stain (B) is standard for G-banding in karyotyping. Rationale: Giemsa produces dark (G-positive) and light bands, widely used for chromosome identification. Quinacrine (Q-banding) and trypsin (pre-treatment) are less common alone.

Correct Answer: C

Rationale: Trisomy 21 (Down syndrome, C) is most survivable. Rationale: Edwards (18) and Patau (13) are often lethal early; Turner (45, X) is viable but less common than Down syndrome, which has milder effects and higher live birth rates.

Correct Answer: C

Rationale: SRY gene (C) causes 46, XX males. Rationale: SRY translocation from Y to X or autosome in XX individuals triggers male development despite no Y chromosome. Turner is 45, X; Rb is unrelated (retinoblastoma).