Transient (Toxic) Synovitis
Abscess
Cellulitis
Primary rheumatologic disorder (i.e. vasculitis)
Iatrogenic
Reactive Arthritis (Poststreptococcal)
Consider if patient has Sickle Cell (fever and limited joint ROM)
Osteomyelitis typically has neither
Osteomyelitis typically has neither
Acute osteoarthritis
Avascular necrosis
Crystal-induced (Gout, Pseudogout)
Gonococcal arthritis, arthritis-dermatitis syndrome
Nongonococcal<unk> 
Lyme disease
Malignancy (metastases, osteochondroma, osteoid osteoma)
Reactive poststreptococcal arthritis
Trauma-induced arthritis
Fracture
Ligamentous injury
Overuse
Avascular necrosis
Decompression sickness
Spontaneous osteonecrosis
Hemorrhagic (e.g. hemophilia, systemic anticoagulation
Seronegative spondyloarthropathies (ankylosing spondylitis, IBD, psoriatic arthritis, reactive arthritis
RA, SLE<unk> rcoidosis, amyloidosis
Periarticular pathology
Cellulitis
Tendonitis
Bursitis
Transient (Toxic) Synovitis (Hip)
Slipped Capital Femoral Epiphysis (SCFE)
Legg Calve Perthes Disease
Fracture
Ligamentous injury
Overuse
Cellulitis
Tendonitis
Bursitis
Septic arthritis
Transient 
Monoarticular arthritis
Abscess
Reactive arthritis
Cellulitis
Septic bursitis
Primary rheumatologic disorder 