**Core Concept**
Courvoisier's law states that an enlarged gallbladder in the absence of gallstones suggests an obstruction of the bile duct, typically due to a malignancy. However, this law has exceptions, particularly when there's an obstruction of the bile duct that doesn't involve a tumor.

**Why the Correct Answer is Right**
Double impaction refers to the simultaneous obstruction of the common bile duct by a gallstone and the cystic duct by another gallstone. This situation prevents the gallbladder from emptying, leading to its enlargement, despite the presence of gallstones. This is an exception to Courvoisier's law because it involves an obstruction of the bile duct due to gallstones, rather than a tumor.

**Why Each Wrong Option is Incorrect**
**Option B:** Portal lymphadenopathy is not an exception to Courvoisier's law. It can be associated with various conditions, including malignancies, but it's not a specific exception to the law.
**Option C:** Periampullary carcinoma is a type of malignancy that can cause an obstruction of the bile duct, which would be in line with Courvoisier's law, not an exception.

**Clinical Pearl / High-Yield Fact**
When diagnosing an enlarged gallbladder, it's essential to consider Courvoisier's law and its exceptions. Double impaction is a critical exception to remember, as it can lead to an enlarged gallbladder in the presence of gallstones, rather than a malignancy.

**✓ Correct Answer: A. Double impaction**