Primary adenocarcinoma was diagnosed by lung biopsy - stage IVb due to multiple extra-thoracic metastases (bone and adrenal). Metastatic destruction of the subarticular medial femoral condyle is the likely cause of his acute knee pain.

Layered periosteal new bone is typical of hypertrophic osteoarthropathy, a recognized paraneoplastic effect of lung cancer. 

Omission errors and satisfaction of search: Initially it was assumed that the pain was due to HOA or inflammation in the knee joint. The metastasis was missed due to failure of perception (omission error) and the presence of multiple abnormalities (satisfaction of search).