Breast Cancer: This is the most common cause of axillary lymph node metastasis with an unknown primary source. The proximity of the breast tissue to the axilla and the frequency of breast cancer make it a prime consideration.

Lung Cancer: Given the lymphatic drainage of the lungs, it's plausible for lung cancer to metastasize to the axillary nodes, especially if the primary tumor is located in the upper lobes.
Gastrointestinal Cancers (e.g., Stomach, Pancreas): Although less common, cancers of the gastrointestinal tract can metastasize to the axillary lymph nodes, particularly if they have spread extensively.
Melanoma: Malignant melanoma can metastasize to any lymph node, including those in the axilla, and may present with an unknown primary if the original melanoma site is small or has regressed.

Thyroid Cancer: Though rare, thyroid cancer can metastasize to axillary lymph nodes. Missing this diagnosis could lead to delayed treatment of a potentially curable cancer.
Testicular Cancer: Some testicular cancers, especially those with a retroperitoneal lymph node spread, can unusually metastasize to the axillary nodes. This diagnosis is critical due to the potential for cure with appropriate treatment.
Sarcomas: Soft tissue sarcomas can metastasize to lymph nodes, including those in the axilla. Early diagnosis is crucial for treatment planning and prognosis.

Prostate Cancer: Prostate cancer rarely metastasizes to axillary lymph nodes, but it should be considered, especially in older males with other risk factors for prostate cancer.
Renal Cell Carcinoma: Similar to prostate cancer, renal cell carcinoma can metastasize to unusual sites, including the axillary lymph nodes, though this is exceedingly rare.
Carcinoids and Neuroendocrine Tumors: These tumors can metastasize to lymph nodes, including those in the axilla, and may present with an occult primary, making them a rare but interesting consideration in the differential diagnosis.