Okay, let's tackle this question about cystic fibrosis and the chromosome involved. The user provided the question but didn't fill in the options or the correct answer. Hmm, but the correct answer is supposed to be provided. Wait, the user included a correct answer in the example, maybe they want me to follow that structure.

First, I need to recall where the cystic fibrosis gene is located. Cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene. I remember that the CFTR gene is located on chromosome 7. Let me double-check that. Yes, chromosome 7, specifically the long arm, which is denoted as 7q. The exact location is 7q31.2. So the mutation is on the long arm of chromosome 7.

Now, the question is asking which chromosome's long arm has the mutation. The correct answer would be chromosome 7. The options A to D aren't given, but typical distractors might be other chromosomes associated with other genetic disorders. For example, Down syndrome is on chromosome 21, sickle cell anemia on 11, and Tay-Sachs on 15. So the incorrect options might be those numbers.

In the explanation, the core concept is the location of the CFTR gene. The correct answer is 7q because that's where the gene is. The wrong options are other chromosomes where other diseases are located. The clinical pearl is to remember that CFTR is on 7q and that it's an autosomal recessive disorder. The correct answer line should be the one that states chromosome 7.

**Core Concept**
Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the **CFTR gene**, located on the **long arm (q arm) of chromosome 7**. This gene encodes a chloride channel critical for epithelial fluid homeostasis.

**Why the Correct Answer is Right**
The **CFTR gene** is specifically localized to **chromosome 7q31.2**. Mutations in this gene disrupt chloride and sodium transport across epithelial cells, leading to viscous mucus in lungs, pancreas, and other organs. The most common mutation, **ΔF508**, causes defective CFTR protein folding and degradation.

**Why Each Wrong Option is Incorrect**
**Option A:** Chromosome 5 (e.g., *HBB* gene for β-thalassemia) is incorrect.
**Option B:** Chromosome 11 (e.g., *α-globin* genes for α-thalassemia) is incorrect.
**Option C:** Chromosome 15 (e.g., *HEXA* gene for Tay-Sachs disease) is incorrect.

**Clinical Pearl / High-Yield Fact**
Never forget: **CFTR = chromosome 7q**. This is a classic NEET PG/USMLE trap—students may confuse it with other single-gene disorders like sickle cell (chromosome 11) or Tay-Sachs (chromosome 15).

**Correct Answer: C. Chromosome 7**

✓ Correct Answer: A. 7