Okay, let's tackle this question about the most commonly mutated gene in ADPKD. Hmm, I remember that ADPKD is a genetic disorder, right? It's one of the more common hereditary kidney diseases. The question is asking which gene is most frequently involved.

First, I need to recall the genes associated with ADPKD. I think there are two main genes involved. There's PKD1 and PKD2. Which one is more common? I believe PKD1 is the primary one. Because I remember that mutations in PKD1 account for the majority of cases, maybe around 85%, while PKD2 is responsible for the remaining 15%.

Wait, what's the function of these genes? PKD1 codes for polycystin-1, a large transmembrane protein, and PKD2 for polycystin-2, which is a calcium channel. They work together in the cell, probably regulating cell growth and fluid flow. When these genes are mutated, the kidneys develop cysts, leading to the disease's symptoms.

So the options here are probably A to D, but they aren't listed. The correct answer should be PKD1. Let me check why the other options are wrong. If the options include genes like COL4A5 (Alport syndrome), CFTR (cystic fibrosis), or others, those are incorrect. Also, maybe CFTR is a distractor here. Another possibility is HNF1B, but that's for maturity-onset diabetes of the young type 5. So each wrong option would be a different gene mutation causing other diseases.

The clinical pearl here is to remember that PKD1 is the main gene for ADPKD, and PKD2 is less common. Also, differentiating ADPKD from other cystic kidney diseases like ARPKD, which is caused by PKHD1, is important. So the key point is the frequency of PKD1 mutations in ADPKD.

**Core Concept**
Autosomal dominant polycystic kidney disease (ADPKD) is primarily caused by mutations in the *PKD1* gene. This gene encodes polycystin-1, a transmembrane protein critical for regulating cell proliferation and fluid balance in renal tubules. Approximately 85% of ADPKD cases involve *PKD1* mutations, with *PKD2* (encoding polycystin-2) accounting for ~15% of cases.

**Why the Correct Answer is Right**
The *PKD1* gene is most commonly mutated in ADPKD, leading to dysfunctional polycystin-1. This protein interacts with polycystin-2 (encoded by *PKD2*), a calcium-permeable channel, to modulate intracellular signaling pathways. Loss of function in *PKD1* disrupts ciliary signaling and promotes cyst formation in renal tubules. Genetic testing confirms diagnosis, and *PKD1* mutations are associated with earlier onset and more severe disease compared to *PKD2*.

**Why Each Wrong Option is Incorrect**
**Option A:** *COL4A5* mutations cause Alport syndrome, a hereditary glomerulonephritis with hematuria and hearing loss.
**Option B:** *CFTR* mutations are responsible for cystic fibrosis, affecting

✓ Correct Answer: A. PKD1