Okay, let's tackle this question about the chromosome associated with cystic fibrosis. First, I need to recall the genetic basis of cystic fibrosis. I remember that CF is an autosomal recessive disorder, which means it's caused by mutations in a specific gene. The gene in question here is the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Now, where is this gene located?

The CFTR gene is located on chromosome 7. I think it's on the long arm, specifically 7q31.2. This is a key point because many genetic disorders are linked to specific chromosomes, and knowing this helps in diagnosing and understanding the inheritance patterns.

Looking at the options, the correct answer should be chromosome 7. The other options might be other chromosomes that are associated with different conditions. For example, Down syndrome is on chromosome 21, sickle cell anemia on 11, and Tay-Sachs on 15. So if those were the distractors, they would be incorrect for this question.

I should also consider why the other options are wrong. For instance, if an option lists chromosome 11, that's where the beta-globin gene is, related to sickle cell. Chromosome 15 might be for Tay-Sachs. Chromosome 21 is for Down. So each of these is a different disorder and thus incorrect here.

The clinical pearl here is that CFTR gene mutations on chromosome 7 lead to defective chloride channels, causing the thick mucus characteristic of CF. This is a high-yield fact for exams like NEET PG or USMLE. Students should remember that chromosome 7 is key here.

Now, structuring the explanation with the required sections. Make sure each part is concise and covers the necessary points without being too lengthy. Check the character count to stay within the limit. Also, use bold for section headers and correct terms. Avoid any markdown beyond what's specified.

Double-check that all sections are included: Core Concept, Why Correct is Right, Why Wrong are Wrong, Clinical Pearl, and Correct Answer Line. Ensure that the explanation clearly differentiates the correct answer from the others and provides a solid understanding of the genetic basis of cystic fibrosis.

**Core Concept**
Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the **CFTR gene**, which encodes a chloride channel. This gene is located on **chromosome 7**. The defective CFTR protein leads to thick, viscous mucus in multiple organs, including the lungs and pancreas.

**Why the Correct Answer is Right**
The **CFTR gene** resides on **chromosome 7 (7q31.2)**. Mutations in this gene disrupt chloride ion transport across epithelial surfaces, causing the hallmark mucus buildup in CF. Over 2,000 mutations in CFTR have been identified, with **ΔF508** being the most common. This genetic defect directly links CF to chromosome 7.

**Why Each Wrong Option is Incorrect**
**Option A:** Chromosome 21 is associated with Down syndrome (trisomy 21).
**Option B:** Chromosome 11 houses the beta-globin gene (sickle cell anemia).
**Option C:**

✓ Correct Answer: A. 7