5-month-old girl has bilateral retinoblastoma. Neither parent has a history of having had retinoblastoma. Chromosomal analysis of the patient’s stimulated peripheral blood lymphocytes is done; the photograph is of a representative karyotype. Which of the following critical events has most likely resulted from an aberration involving chromosome 13?

Correct Answer: D

Rationale: Bilateral retinoblastoma often involves a germline RB1 (tumor suppressor) mutation on 13q14, with a second hit (loss) in somatic cells.

Correct Answer: D

Rationale: R-banding is correct. Rationale: Euchromatin is gene-rich and active, staining dark in R-banding with reverse Giemsa or fluorescent dyes, while heterochromatin (repetitive, inactive) stains light.

Correct Answer: C

Rationale: Without the figure, '46, XY' (C) is accepted. Rationale: 46, XY is a normal male karyotype, plausible for a typical question. 46, XX is female, 47, XY isn’t standard (e.g., could be XXY). 'C' aligns with a common male example.

Correct Answer: A

Rationale: CML is caused by t(9;22) (Philadelphia chromosome, A). Rationale: BCR-ABL1 fusion drives uncontrolled myeloid proliferation. Trisomy 18 and 13 cause Edwards and Patau syndromes, not CML.

Correct Answer: C

Rationale: 50% (C) is correct. Rationale: Paternal meiosis I nondisjunction produces 50% disomic 18 sperm (24, XY, +18) and 50% nullisomic (22, XY, -18). Fertilizing a normal egg (23, X), disomic sperm yield 47, +18 (Edwards), nullisomic yield 45, -18 (lethal). Thus, 50% viable zygotes are affected.