Most Likely Diagnosis: Cancer of the Head of the Pancreas
The most likely cause is cancer of the head of the pancreas (Option D), given the combination of obstructive jaundice, markedly elevated CA19-9 (>500 U/mL), hugely dilated common bile duct and gallbladder, and the clinical presentation of painless jaundice with a palpable gallbladder (Courvoisier's sign).
Clinical Reasoning
Key Diagnostic Features Pointing to Pancreatic Head Cancer
CA19-9 >500 U/mL is highly suggestive of malignancy: While CA19-9 can be elevated in benign biliary obstruction, levels persistently >100 U/mL after biliary decompression strongly suggest malignancy, and CA19-9 does not discriminate between cholangiocarcinoma, pancreatic, or gastric malignancy 1. The markedly elevated level (>500 U/mL) in this case significantly increases the probability of pancreatic or biliary malignancy 1.
Hugely dilated common bile duct AND gallbladder: This pattern of "double duct dilation" with a palpable, distended gallbladder is classic for distal biliary obstruction from pancreatic head malignancy 1. The gallbladder typically does not become palpably distended in choledocholithiasis due to chronic inflammation and fibrosis.
Obstructive pattern on labs: Elevated total and direct bilirubin with high ALP and GGT indicates extrahepatic biliary obstruction 1. This cholestatic pattern is consistent with malignant obstruction 1.
Why Other Options Are Less Likely
Option A (Mirizzi's Syndrome): This involves extrinsic compression of the common hepatic duct by an impacted gallstone in the cystic duct or Hartmann's pouch. While it causes biliary obstruction, CA19-9 levels >500 U/mL would be unusual, and the gallbladder is typically contracted and fibrotic rather than hugely dilated 2, 3.
Option B (Liver Metastasis): While liver metastases can cause intrahepatic cholestasis and elevated CA19-9, they typically do not cause "hugely dilated" common bile duct and gallbladder unless there is concomitant extrahepatic biliary obstruction 1, 4. The clinical picture better fits extrahepatic obstruction.
Option C (Distal Common Bile Duct Obstruction): This is anatomically correct but too nonspecific. While pancreatic head cancer causes distal CBD obstruction, this option doesn't identify the underlying etiology. Given the CA19-9 >500 U/mL and clinical presentation, the specific cause (pancreatic head malignancy) should be identified 1.
Important Clinical Caveats
CA19-9 Interpretation
CA19-9 can be elevated in benign biliary obstruction: Levels can reach >1000 U/mL in choledocholithiasis and cholangitis, particularly when associated with elevated bilirubin, ALP, and GGT 2, 3, 5. However, CA19-9 levels typically normalize rapidly after relief of benign obstruction 2, 3.
Sensitivity and specificity considerations: CA19-9 >100 U/mL has 75% sensitivity and 80% specificity for cholangiocarcinoma in PSC patients, but it is elevated in up to 85% of cholangiocarcinoma patients overall 1. The correlation with bilirubin, ALP, and GGT levels (r = 0.69-0.83) means that obstructive jaundice itself can drive CA19-9 elevation 2.
Next Steps in Management
Imaging with CT or MRI/MRCP is essential: While ultrasound identified the dilated ducts, contrast-enhanced CT or MRI with MRCP is needed to define the pancreatic mass, assess resectability, and evaluate for metastases 1.
Tissue diagnosis required: Despite the highly suggestive clinical picture, histopathological confirmation via endoscopic ultrasound with fine-needle aspiration (EUS-FNA) or ERCP with brushings is necessary before definitive treatment 1.
Biliary decompression may be needed: If the patient has cholangitis or severe jaundice, ERCP with stenting or percutaneous transhepatic biliary drainage may be required before definitive surgery 1, 6.