Tuberculosis (TB): TB is a common cause of lymphadenopathy, especially in the neck, and can also affect the lower limbs. The presence of lymphadenopathy in both the neck and lower limb could be indicative of disseminated TB.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can cause lymphadenopathy in multiple regions, including the neck and lower limbs.
Infections (e.g., Staphylococcus aureus, Streptococcus pyogenes): Bacterial infections can cause localized lymphadenopathy, but in some cases, especially if the infection is severe or becomes systemic, it can lead to lymphadenopathy in multiple areas.
Metastatic Cancer: Certain types of cancer, such as squamous cell carcinoma of the head and neck or melanoma, can metastasize to lymph nodes in both the neck and lower limbs.

HIV/AIDS: Although less common, HIV/AIDS can cause generalized lymphadenopathy, including in the neck and lower limbs. Missing this diagnosis could have significant implications for treatment and prognosis.
Systemic Lupus Erythematosus (SLE): SLE is an autoimmune disease that can cause lymphadenopathy among other systemic symptoms. It's crucial not to miss this diagnosis due to its potential for serious complications if left untreated.
Cat-Scratch Disease: Caused by Bartonella henselae, this disease typically causes localized lymphadenopathy near the site of the cat scratch but can occasionally cause more widespread lymphadenopathy.

Kikuchi-Fujimoto Disease: A rare, self-limiting condition that primarily affects the lymph nodes in the neck but can occasionally cause lymphadenopathy in other areas.
Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can cause lymphadenopathy. It can be localized or multicentric.
Whipple's Disease: A rare, systemic bacterial infection (Tropheryma whipplei) that can cause lymphadenopathy among other symptoms like malabsorption and weight loss.