## Core Concept
The patient's presentation of jaundice, abdominal pain, dark-colored urine, and a palpable but non-tender gallbladder suggests a condition affecting the biliary system, likely involving obstruction. The key clinical features here point towards a diagnosis related to the gallbladder and bile ducts.

## Why the Correct Answer is Right
The correct answer, **C. Carcinoma of the pancreas**, is suggested by Courvoisier's law, which states that in the presence of a palpable, non-tender gallbladder accompanied by painless jaundice, the cause is more likely to be a malignant obstruction, such as pancreatic cancer, rather than gallstones. This is because pancreatic cancer often obstructs the common bile duct, leading to jaundice and can cause the gallbladder to become palpable due to distension. The patient's symptoms of tiredness, abdominal pain, jaundice, and dark-colored urine (indicative of bilirubinuria) align with obstructive jaundice.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, conditions like cholecystitis or cholelithiasis (gallstones) would present with a tender gallbladder, not a non-tender one.
- **Option B:** This option is also not provided, but for instance, if it were a condition like hepatitis, we would expect the patient to have liver enzyme elevations and possibly a tender liver, but not typically a palpable gallbladder.
- **Option D:** Similarly, without the specific option, if we consider other causes like cholangiocarcinoma, while it could present with obstructive jaundice, the specific combination of findings (especially the palpable gallbladder) and the context provided make pancreatic carcinoma a more fitting diagnosis based on the information given.

## Clinical Pearl / High-Yield Fact
A key clinical pearl from this case is Courvoisier's law, which helps differentiate between causes of obstructive jaundice. Remembering that a **palpable, non-tender gallbladder with jaundice is more suggestive of a malignant cause (like pancreatic cancer) rather than a benign cause (like gallstones)**, can help in making a diagnosis.

## Correct Answer Line
**Correct Answer: C. Carcinoma of the pancreas**