• "Greenish" is an uncommon colour for a swollen limb. The main causes are myeloperoxidase-rich tumour (chloroma), pyocyanin from Pseudomonas aeruginosa, and an old bruise (biliverdin).
• "Tissue overgrowth inside the kidneys" is not a specific term. It could mean a renal mass, infiltrate, hyperplasia or fibrosis, so the differential below is built around it.

Differential diagnosis

A. Unifying diagnoses (one process explains both the leg and the kidney)

1. Myeloid sarcoma (chloroma) with or without acute or chronic myeloid leukaemia.
   • The name comes from the green colour of fresh lesions, caused by myeloperoxidase (Andrews' Diseases of the Skin). Dermatology texts also list chloroma under green lesions, alongside Pseudomonas infection (Dermatology 2-Volume Set 5e).
   • The kidney can be involved too. Recent case reports and a 2025 multi-institutional series describe myeloid sarcoma of the kidney and genitourinary tract.
   • It is the best fit if the renal "overgrowth" is a leukaemic infiltrate.
2. Lymphoma or other leukaemia with leukaemia cutis or soft-tissue infiltrate. This is common in the elderly and can infiltrate both the kidney and the limb.
3. Metastatic or multifocal malignancy. Renal cell carcinoma or another primary can metastasise to soft tissue and cause a necrotic, discoloured swelling.
4. Kaposi sarcoma. It causes violaceous-brown to bruise-like lower-limb swelling and lymphoedema, and can occur in immunosuppressed patients (for example after a renal transplant). Visceral spread is possible.
5. Erdheim-Chester disease, IgG4-related disease, or retroperitoneal fibrosis.
   • These can produce "fibrous overgrowth" around or in the kidney (perinephric infiltration, mass-like lesions, obstructive nephropathy). Goldman-Cecil lists Erdheim-Chester and Castleman disease as mimics of retroperitoneal fibrosis, and Brenner and Rector describes mass-like kidney lesions in IgG4 disease.
   • Obstruction or venous or lymphatic compression can cause leg swelling.
   • Erdheim-Chester also causes long-bone pain in the legs.

B. Leg-focused diagnoses (explain the green colour)

1. Pseudomonas aeruginosa soft-tissue infection or cellulitis.
   • Pyocyanin gives a blue-green pigment, as in green nail syndrome (Fitzpatrick's Dermatology).
   • Risk factors are age, diabetes, venous ulcers, peripheral vascular disease, and wounds or dressings.
   • Look for ecthyma gangrenosum and necrotizing infection.
2. Necrotizing fasciitis or gas gangrene (clostridial). This must be excluded early. Look for pain out of proportion, bullae, crepitus, and systemic toxicity.
3. Resolving haematoma or ecchymosis. A large bruise turns green over days, especially on anticoagulants or antiplatelets, or with uraemic coagulopathy or thrombocytopenia. It is common at this age.
4. Deep vein thrombosis, post-thrombotic syndrome, or venous stasis. Haemosiderin and bruising can colour the skin. Lower-limb DVT is also more likely in malignancy and nephrotic states.
5. Acute limb ischaemia or gangrene (wet gangrene). Peripheral vascular disease is likely in an 84-year-old, and secondary infection can turn tissue dark green or black.
6. Compartment syndrome or ruptured Baker's cyst. These give a tense swelling with later bruising.

C. Renal-focused diagnoses to explain the "overgrowth"

• Renal cell carcinoma, or a renal mass with a tumour thrombus in the renal vein or IVC. A thrombus can cause bilateral or unilateral leg oedema.
• Urothelial carcinoma, renal lymphoma, or amyloid or other infiltrative disease.
• Benign lesions, for example angiomyolipoma or oncocytoma.
• Nephrotic syndrome with hypoalbuminaemia, and renal failure with fluid overload. Both cause leg oedema but not a green colour, and are usually bilateral.

Suggested work-up in the emergency department

• Immediate: vitals and sepsis screen, CBC with differential and peripheral smear (blasts), coagulation profile, renal and liver function, lactate, CRP, CK, blood cultures.
• Leg:
  • Wound or tissue swab for culture, and a Wood lamp if available (pyoverdin fluoresces).
  • Lower-limb Doppler ultrasound.
  • Urgent surgical review if necrotizing infection is suspected.
  • Biopsy for histology and immunohistochemistry (MPO, CD68, CD117, CD43) and flow cytometry, with a skin punch biopsy if a mass or nodule is present.
• Kidney: review the prior imaging and biopsy. Do contrast CT or MRI of the abdomen and pelvis, and look for renal vein or IVC thrombus, retroperitoneal disease, and lymph nodes.
• Systemic: serum IgG4, LDH, urinalysis and protein, and bone marrow biopsy with cytogenetics if blasts are present.

Caveats and next steps

• This is a differential based on a short description. I did not find a textbook case that matches exactly.
• Please tell me what "tissue overgrowth in the kidneys" refers to (biopsy result, imaging, or a named diagnosis). It would change the ranking substantially.
• Also useful: anticoagulant use, diabetes, transplant or immunosuppression, fever, pain, skin breaks, and the blood counts.
• Treat as infection or necrotizing fasciitis until proven otherwise if he is febrile, toxic, or in severe pain. That is the time-critical diagnosis.