Question: Crystal arthropathy is a class of joint disorder (called "arthropathy"), which acid is characterized by accumulation of tiny crystals in one or more joints. Polarizing microscopy and application of other crystallographic techniques have improved identification of different microcrystals including monosodium urate, what is the heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula CHNO?

Context paragraphs:
Crystal arthropathy: Crystal arthropathy is a class of joint disorder (called "arthropathy") that is characterized by accumulation of tiny crystals in one or more joints. Polarizing microscopy and application of other crystallographic techniques have improved identification of different microcrystals including monosodium urate, calcium pyrophosphate dihydrate, calcium hydroxyapatite, and calcium oxalate.

Tophus: A tophus (Latin: "stone", plural tophi) is a deposit of uric acid crystals, in the form of monosodium urate crystals, in people with longstanding hyperuricemia (high levels of uric acid in the blood).  Tophi are pathognomonic for the disease gout.  Most people with tophi have had previous attacks of acute arthritis, eventually leading to the formation of tophi.  Chronic tophaceous gout is known as Harrison Syndrome.

TEMPO: (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl, commonly known as TEMPO, is a chemical compound with the formula (CH)(CMe)NO.  This heterocyclic compound is a red-orange, sublimable solid.  As a stable radical, it has applications in chemistry and biochemistry.  TEMPO was discovered by Lebedev and Kazarnowskii in 1960.  It is prepared by oxidation of 2,2,6,6-tetramethylpiperidine.  TEMPO is widely used as a radical marker, as a structural probe for biological systems in conjunction with electron spin resonance spectroscopy, as a reagent in organic synthesis, and as a mediator in controlled radical polymerization.  The stability of this radical is attributed to the resonance provided by non-bonding electrons on the nitrogen atom, which form a 2c3e (half-) bond between nitrogen and oxygen, and hyperconjugative ability.  Additional stability arises from the steric protection provided by the four methyl groups adjacent to the nitroxyl group; however, the methyl groups prevent a double bond occurring between either carbon adjacent to nitrogen.  The stability of the radical is also indicated by the weakness of the O–H bond in the hydrogenated derivative TEMPO–H. With an O–H bond dissociation energy of about 70 kcal/mol, this bond is about 30% weaker than a typical O–H bond.

Uric acid: Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula CHNO.  It forms ions and salts known as urates and acid urates, such as ammonium acid urate.  Uric acid is a product of the metabolic breakdown of purine nucleotides, and it is a normal component of urine.  High blood concentrations of uric acid can lead to gout and are associated with other medical conditions including diabetes and the formation of ammonium acid urate kidney stones.

Jaccoud arthropathy: Jaccoud arthropathy (JA), Jaccoud deformity or Jaccoud's arthopathy is a chronic non-erosive reversible joint disorder that may occur after repeated bouts of arthritis.  It is caused by inflammation of the joint capsule and subsequent fibrotic retraction, causing ulnar deviation of the fingers, through metacarpophalangeal joint (MCP) subluxation, primarily of the ring and little-finger.  Joints in the feet, knees and shoulders may also get affected.  It is commonly associated with systemic lupus erythematosus (SLE), and occurs in roughly 5% of all cases.

N-Hydroxypiperidine: "N"-Hydroxypiperidine (also known as 1-piperidinol and 1-hydroxypiperidine) is the chemical compound with formula CHNO.  It is a hydroxylated derivative of the heterocyclic compound piperidine.

Allantoin: Allantoin is a chemical compound with formula CHNO.  It is also called 5-ureidohydantoin or glyoxyldiureide.  It is a diureide of glyoxylic acid.  Allantoin is a major metabolic intermediate in most organisms including animals, plants and bacteria.  It is produced from uric acid, which is a degradation product of purine nucleobases, by urate oxidase (or uricase).

Nitronate: A nitronate (IUPAC: azinate) in organic chemistry is a functional group with the general structure 2− .  It is the anion of a nitronic acid, a tautomeric form of a nitro compound.  Just as ketones and aldehydes exist in equilibrium with their enol tautomer in basic and acidic conditions, nitro compounds exist in equilibrium under basic conditions with their nitronate tautomer.  The base deprotonates the α carbon, or the carbon directly attached to the nitrogen.  The nitronate has two different resonance structures, one with a negative charge on the α carbon and a double bond between the nitrogen and one of the oxygens, and another resonance structure with a double bond between the nitrogen and the α carbon, and no double bond between the nitrogen and the oxygens.  A nitronic acid is also called an aci form.  In the Nef reaction nitronic acids are degraded to ketones.  They can be alkylated on oxygen and used as a dipole in 1,3-dipolar cycloadditions.

Pyridine-N-oxide: Pyridine-"N"-oxide is the heterocyclic compound with the formula CHNO.  This colourless, hygroscopic solid is the product of the oxidation of pyridine.  It was originally prepared using peracids as the oxidising agent.  The molecule is planar.  The compound is used infrequently as an oxidizing reagent in organic synthesis.  It also serves as a ligand in coordination chemistry, and to activate the pyridine ring towards aromatic electrophilic substitution.

Azetidine-2-carboxylic acid: Azetidine-2-carboxylic acid (abbreviated Aze) is a plant non-protein amino acid homologue of proline with the molecular formula CHNO.  Aze is a heterocyclic, 4 membered ring with nitrogen as its heteroatom (an azetidine), and a carboxylic acid group substituted on one of the ring carbon atoms.  The main difference between Aze and proline is the ring of Aze has four members and the ring of proline has five.  Aze has the ability to act as an analog of proline and can be incorporated into proteins in place of proline.

Supporting paragraphs: Crystal arthropathy Uric acid
Answer: Uric acid