Most common type of class II mutation in cystic fibrosis is
**Core Concept**
Cystic fibrosis (CF) is a genetic disorder caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. Class II mutations in CF involve deletions or insertions in the CFTR gene that affect splicing. The most common type of class II mutation in CF is a three-nucleotide deletion at codons 507-508 (ΞF508).
**Why the Correct Answer is Right**
The ΞF508 mutation is a type of class II mutation that involves a deletion of three nucleotides (GTT) at codons 507-508 in the CFTR gene. This deletion leads to the removal of a phenylalanine residue and results in a truncated protein that is unable to reach the cell surface. The ΞF508 mutation accounts for approximately 70% of CF cases and is associated with severe pulmonary and pancreatic manifestations of the disease. The defective CFTR protein is retained in the endoplasmic reticulum and undergoes premature degradation, leading to a significant reduction in chloride transport across epithelial cells.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so we cannot evaluate its correctness.
**Option B:** This option is not specified, so we cannot evaluate its correctness.
**Option C:** This option is not specified, so we cannot evaluate its correctness.
**Option D:** This option is not specified, so we cannot evaluate its correctness.
**Clinical Pearl / High-Yield Fact**
The ΞF508 mutation is the most common cause of cystic fibrosis and is associated with severe pulmonary and pancreatic manifestations. Treatment strategies for CF, including pharmacological agents and gene therapy, are being developed to target the underlying defect in CFTR function.
**Correct Answer: D. ΞF508 deletion.**