**Core Concept**
The G551D mutation is a specific genetic alteration in the CFTR (cystic fibrosis transmembrane conductance regulator) gene, leading to defective chloride channel function in the lungs and other organs. Treatment for cystic fibrosis involves correcting this defect to restore chloride channel function and improve lung function.

**Why the Correct Answer is Right**
The correct answer is a medication that targets the defective CFTR protein by increasing its activity through a small molecule corrector. This corrector stabilizes the CFTR protein at the cell surface, allowing it to function properly and improving chloride channel function. The medication is specifically designed to address the G551D mutation, which is a common cause of cystic fibrosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a small molecule corrector for the CFTR protein. Instead, it is a different class of medication that targets a different aspect of cystic fibrosis pathophysiology.
**Option B:** This option is incorrect because it is not approved for the treatment of cystic fibrosis with the G551D mutation. It may be used for a different indication or mutation.
**Option C:** This option is incorrect because it is not a small molecule corrector for the CFTR protein. It may be a different class of medication or have a different mechanism of action.

**Clinical Pearl / High-Yield Fact**
The G551D mutation is one of several mutations that can be targeted by specific treatments for cystic fibrosis. This highlights the importance of genetic testing in guiding treatment decisions for patients with cystic fibrosis.

**Correct Answer:** C. Ivacaftor (Kalydeco)