Gene therapy feasibility — Which of the following genetic diseases is most amenable to correction by gene engineering because it is caused by a single defective gene?

Introduction:
Gene engineering approaches are best suited to monogenic disorders in which correcting or replacing a single gene can restore function. This question asks you to select the disease that fits the single-gene paradigm most directly in clinical gene therapy practice.

Given Data / Assumptions:

• Cystic fibrosis results from mutations in the CFTR chloride channel gene.
• Down's syndrome and Cri du Chat involve chromosome-scale copy-number changes.
• Duchenne muscular dystrophy is monogenic but presents delivery and size challenges (very large dystrophin gene).

Concept / Approach:
Prioritize clear monogenic etiology and realistic vector packaging. CFTR, though sizable, has been the focus of many gene therapy trials; in contrast, trisomies or large deletions cannot be easily corrected by adding a single gene copy. Dystrophin therapy often requires mini-/micro-dystrophin constructs and faces muscle-wide delivery hurdles.

Step-by-Step Solution:

Verification / Alternative check:
Clinical pipelines include CFTR modulators and gene therapy/editing research; chromosomal-number disorders remain refractory to current correction strategies.

Why Other Options Are Wrong:

Common Pitfalls:
Equating “genetic disease” with “good gene therapy target” without considering genetic architecture and vector capacity.

Final Answer:
Cystic fibrosis (CFTR gene defect).