**Core Concept**
The patient's presentation of obstructive jaundice, weight loss, and dilated common bile duct (CBD) and main pancreatic duct (MPD) with a normal pancreas suggests a diagnosis related to bile duct and pancreatic duct obstruction. This clinical scenario is characteristic of a condition where a tumor develops in the head of the pancreas, leading to obstruction of both the CBD and MPD.

**Why the Correct Answer is Right**
The correct diagnosis is ampullary carcinoma or more commonly, pancreatic head carcinoma (adenocarcinoma). The tumor develops in the head of the pancreas, causing obstruction of the CBD and MPD, resulting in jaundice and weight loss. The normal pancreas on CT scan rules out other pancreatic pathologies such as chronic pancreatitis or pancreatic pseudocysts. The dilated CBD and MPD are characteristic findings in pancreatic head carcinoma, which is known to cause obstructive jaundice.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because ampullary carcinoma is a possibility, but it is less common than pancreatic head carcinoma. Ampullary carcinoma also causes obstruction of the CBD and MPD, but it arises from the ampulla of Vater, whereas pancreatic head carcinoma arises from the pancreas itself.

**Option B:**
This option is incorrect because choledocholithiasis (gallstones in the CBD) can cause obstructive jaundice, but it would not dilate the MPD. Additionally, the patient's weight loss and the dilated MPD suggest a more complex pathology than a simple gallstone.

**Option C:**
This option is incorrect because pancreatic pseudocysts are fluid collections within the pancreas, which do not cause dilatation of the CBD or MPD. Pancreatic pseudocysts can cause obstructive jaundice if they compress the CBD, but the patient's presentation and imaging findings suggest a more solid tumor.

**Option D:**
This option is incorrect because chronic pancreatitis can cause dilatation of the MPD, but it would not cause obstructive jaundice due to a tumor. Chronic pancreatitis would also likely show abnormalities in the pancreas on CT scan, which is not the case in this patient.

**Clinical Pearl / High-Yield Fact**
Pancreatic head carcinoma is a common cause of obstructive jaundice, and it often presents with weight loss and dilated CBD and MPD. The normal pancreas on CT scan in this patient's case is an important clue, as it rules out other pancreatic pathologies.

**Correct Answer:** C.