Septic Arthritis: Given the patient's age, inflammatory ankle pain, significantly elevated CRP (>50), and increased WBC count (14.8 x10^9/L) with a high neutrophil absolute count (10.8 x10^9/L), septic arthritis is the most likely diagnosis. The high neutrophil count suggests a bacterial infection.

Gout: Although the presentation is more suggestive of septic arthritis, gout can cause significant inflammatory pain and elevated inflammatory markers. The absence of a clear history of gout or urate crystals in the joint fluid does not rule out this diagnosis.
Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): Similar to gout, pseudogout can cause acute inflammatory arthritis and should be considered, especially if joint aspiration reveals calcium pyrophosphate dihydrate crystals.
Rheumatoid Arthritis (RA) Flare: While less likely given the acute presentation and the specific lab values, an RA flare could cause significant inflammatory pain and elevated inflammatory markers. However, the neutrophil predominance is more typical of an infectious process.

Osteomyelitis: Although less common than septic arthritis, osteomyelitis (bone infection) can present with similar symptoms and lab findings. It's crucial to consider this diagnosis to avoid missing a potentially serious condition that requires prompt treatment.
Cellulitis with Deep Vein Thrombosis (DVT): Cellulitis can cause significant inflammation and pain, and when combined with DVT, it can mimic septic arthritis. The presence of DVT would significantly alter management.
Lyme Arthritis: In endemic areas, Lyme disease can cause arthritis that might mimic septic arthritis, especially if the patient has not noticed the characteristic "bull's-eye" rash.

Seronegative Spondyloarthropathies (e.g., Psoriatic Arthritis, Ankylosing Spondylitis): These conditions can cause inflammatory arthritis but are less likely given the acute presentation and the patient's age.
Hematogenous Osteonecrosis: Although rare, osteonecrosis (avascular necrosis) can cause acute pain and might be considered in the differential, especially if there's a history of steroid use or other risk factors.
Charcot Arthropathy (Neuropathic Arthropathy): Typically seen in patients with significant neuropathy (e.g., diabetic neuropathy), Charcot arthropathy can cause a destructive arthropathy that might mimic an infectious process.