Ans. b. Uric acid (Ref: Harrison 19/e p2233, 18/e p2837)The underlying basis of gouty arthritis is increased serum uric acid level.Most common joint involved in gout: Meta-Tarso-Phalangeal jointsQPrecipitation of monosodium urate crystalsQ into the joints is the underlying basis for these urate crystals is increased total body urate levels.Diagnosis in a 40 years old man, who presents with acute onset pain and swelling of left great toe and punched out lesion of phalanx and overhanging edges on X-ray is gout. (AIIMS November 2013 repeat)Gout is a metabolic disease that most often affects middle- aged to elderly men and postmenopausal women. It results from an increased body pool of urate with hyperuricemia. It typically is characterized by episodic acute and chronic arthritis caused by deposition of MSU crystals in joints and connective tissue tophi and the risk for deposition in kidney interstitium or uric acid nephrolithiasis. Usually, only one joint is affected initially and the metatarsophalangeal joint of the first toe is the most commonly involved joint. '- Harrison 18/e p2837'Radiographic Features of Gout: Early in the disease radiographic studies may only confirm clinically evident swelling. Cystic changes, well-defined erosions with sclerotic margins (often with overhanging bony edges), and soft tissue masses are characteristic features of advanced chronic tophaceous gout. Ultrasound, CT and MRI are being studied and are likely to become more sensitive for early changes. - Harrison 18/e p2837'Tophi appear as characteristic punched-out cysts or deep erosions in the para-articular bone ends. These excavations are larger and slightly further from the joint margin than the typical rheumatoid erosions. This punched out lesion in gouty arthritis is called Martel's sign, G sign or rat-bite erosion.''Gout: X-ray of feet demonstrated soft tissue swelling, cystic changes, and well-defined 'punched-out' type lytic lesions with sclerotic margins and overhanging bony edges (Martel's sign, G sign or rat-bite erosion). ''Most common joint involved in gout: Meta-Tarso-Phalangeal jointsQ.'GoutA disorder of purine metabolism characterized by hyperuricemia, deposition of monosodium urate-monohydrate crystals in Joints and per-articular tissuesQ and recurrent attacks of acute synovitis.Late changes include cartilage degeneration, renal dysfunction and uric acid urolithiasisMC joint involved in gout: Meta-Tarso-Phalangeal jointsQThe underlying basis of gouty arthritis is increased serum uric acid levelQSerum urate levels >7 mg/dL is defined as hyperuricemiaEpidemiology:Commoner in CaucasiansMore common in menQ, age >30 yearsQStereotype patient is obese, hypertensive and fond of alcohol and may be nudged into an attack by uncontrolled administration of diuretics or aspirinQPathology:Precipitation of monosodium urate crystalsQ into the joints is the underlying basis for these urate crystals is increased total body urate levels.Tophi are nodular deposits of monosodium urate monohydrate crystals, with an associated foreign body reaction.Pathognomonic hallmark of gout: TophiQTophi evolve from repeated precipitation of urate crystals during attackClinical Feature:Acute attack is sudden onset of severe joint pain that lasts for a week or two.Usually comes out of blue but may be precipitated by minor trauma, illness, unaccustomed exercise, alcohol, ACTCH, steroid withdrawal, hypouricemic therapy and drugs.Diagnosis:Definitive diagnosis is made by examination of synovial fluid or tophaceous material with polarized light microscopy and identifying monosodium urate crystals.Crystals have following characteristicsDuring attack: Intracellular and needle shapedQAfter the attack: Extracellular and bluntedQX-ray changes in GoutX-ray of feet demonstrated soft tissue swelling, cystic changes, and well-defined 'punched- out' type lytic lesions with sclerotic margins and overhanging bony edges (Martel's sign, G sign or rat-bite erosion)Q.Treatment of GoutAnti-inflammatoryHypouricemicsXanthine oxidase inhibitors* Mainstay of treatment during acute attack is administration of anti-inflammatory drugs such as colchicine, NSAIDs (except aspirin) or glucocorticoidsQ* Probenecid or sulfinpyrazone can be used if renal function is normal.* Allopurinol, a xanthine oxidase inhibitorQ is usually preferred.These drugs should never be started in acute attack, and they should always be covered by an anti inflammatory preparations or colchicine; otherwise they may actually precipitate an acute attackQ.In chronic tophaceous gout and in all patients with renal complications, allopurinol is drug of choiceQ.