A 68-year-old man with newly diagnosed type 2 diabetes mellitus presents with a 6-week history of progressive jaundice, pruritus, dark urine, and pale stools. On examination, Courvoisier's sign is positive. Contrast-enhanced CT shows the "double duct sign" with a hypoenhancing lesion in the pancreatic head. The resected specimen is shown. The structure marked **A** in the diagram represents the primary pathological lesion. Which of the following best describes the gross and histological characteristics of this lesion?

The structure marked A — the ill-defined firm white-grey mass centred in the pancreatic head — is the hallmark gross appearance of pancreatic ductal adenocarcinoma (PDAC). In this case, the resected specimen confirmed moderately differentiated PDAC with negative resection margins, demonstrating the characteristic desmoplastic stromal reaction that produces the firm, white-grey appearance. The tumour's location in the pancreatic head directly obstructs the intrapancreatic common bile duct (structure B), causing the proximal bile duct dilatation (structure C) and resulting in obstructive jaundice — the "double duct sign" on imaging. This gross morphology, combined with the clinical presentation (painless progressive jaundice, Courvoisier's sign, elevated CA 19-9, and new-onset diabetes) and histological confirmation, is pathognomonic for PDAC. Robbins and Cotran emphasizes that the firm, white-grey, ill-defined mass with marked fibrosis is the classic gross appearance of PDAC, distinguishing it from other pancreatic neoplasms.