**Core Concept**
Cystic fibrosis is an autosomal recessive genetic disorder caused by mutations in the CFTR gene. This means that a person must inherit two defective copies of the gene (one from each parent) to express the disease. The parents of an affected child are usually carriers, meaning they have one normal and one defective copy of the gene.

**Why the Correct Answer is Right**
When the parents are carriers, there is a 25% chance that each child will inherit two defective copies of the gene (one from each parent) and be affected by the disease. This is because each parent has a 50% chance of passing on their normal copy and a 50% chance of passing on their defective copy. Since the parents are both carriers, the probability of each child inheriting two defective copies is calculated as follows:

* 50% chance that the first parent passes on their defective copy (1/2)
* 50% chance that the second parent passes on their defective copy (1/2)
* The probability of both events occurring is 1/2 × 1/2 = 1/4 or 25%

This means that 25% of the time, or 1 in 4, the child will inherit two defective copies and be affected by the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because there is a chance that the sibling will be affected, not zero.

**Option B:** Incorrect because the probability of the sibling being affected is not 1 in 2 (50%).

**Option D:** Incorrect because the probability of the sibling being affected is not 4 in 3 (approximately 1.33).

**Clinical Pearl / High-Yield Fact**
In autosomal recessive conditions, the parents are usually carriers and have a 25% chance of passing on the disease to each child. This is because each parent has a 50% chance of passing on their normal copy and a 50% chance of passing on their defective copy.

**✓ Correct Answer: C. 4-Jan**