The most common cause of trisomy 21:

Correct Answer: B

Rationale: Maternal meiosis I nondisjunction (B) is the most common cause of Down syndrome. Rationale: 90% of trisomy 21 cases are maternal, mostly MI errors (homologs fail to separate), linked to maternal age.

Correct Answer: C

Rationale: 45, XX, t(13;13) (C) is balanced, phenotypically normal. Rationale: Balanced translocation carriers (45 chromosomes) have normal gene dosage; others (XXY, X, +13) show syndromes.

Correct Answer: D

Rationale: Zero (D) chance for an affected female. Rationale: Mother (XX, affected, X^a X^a) and father (XY, unaffected, X^+ Y) produce sons (X^a Y, all affected) and daughters (X^a X^+, carriers). X-linked recessive requires two mutant alleles in females, impossible here.

Correct Answer: C

Rationale: Nits on hair shafts indicate Pediculosis pubis (C, pubic lice). Rationale: Pinworms (A) are intestinal; Molluscum (B) causes skin lesions; Shigella (D) and Streptococcus (E) are bacterial, not associated with nits.

Correct Answer: C

Rationale: Bleeding causes include steroids (A), foreign bodies (B), urethral prolapse (D), and neoplasms (E). Hemorrhagic cystitis (C) causes urinary, not vaginal, bleeding. Rationale: Cystitis affects the bladder; vaginal bleeding requires a genital source.