**Core Concept**
Omeprazole is a proton pump inhibitor (PPI) that inhibits the final step in gastric acid production by irreversibly blocking the H+/K+ ATPase (proton pump) on parietal cells.

**Why the Correct Answer is Right**
Omeprazole binds to the H+/K+ ATPase in a dose-dependent manner, leading to a rapid decrease in gastric acid secretion. The enzyme is irreversibly inhibited, and the effect reaches a peak within 2 hours. The plasma half-life of omeprazole is approximately 0.5-1.5 hours, but its effect on the proton pump lasts longer due to the irreversible binding. The effect plateaus by the 4th day, indicating that the proton pumps have been maximally inhibited.

**Why Each Wrong Option is Incorrect**
* **Option B:** This is incorrect as the normalization of HCl secretion after omeprazole discontinuation is a process that takes longer than a week.
* **Option C:** This is incorrect as the effect of omeprazole on gastric acid secretion does not return to normal within 3 days.
* **Option D:** This is incorrect as the normalization of HCl secretion after omeprazole discontinuation is a gradual process that takes longer than 1 week.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the effects of omeprazole and other PPIs can last for up to 5-7 days after discontinuation due to the irreversible inhibition of the proton pump.

**Correct Answer:** A. 7 days.