why gout affects mainly at 1st metatarsalopharyngeal joint
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gout AND "first metatarsophalangeal"
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gout first metatarsophalangeal joint predilection temperature biomechanics explanation
Gout commonly first attacks the 1st metatarsophalangeal (MTP) joint, the big-toe joint, causing podagra. It is a predilection, not an exclusive site.
Why this joint is favored:
- Cooler peripheral temperature: Monosodium urate is less soluble at lower temperatures. The big toe is distal and relatively cool, especially overnight, so crystals precipitate more easily.
- Mechanical stress and microtrauma: The 1st MTP bears high repetitive load and undergoes marked dorsiflexion during walking. Local physical stress can promote crystal nucleation and trigger inflammation.
- Pre-existing osteoarthritis is common there: Damaged or osteoarthritic cartilage provides a favorable surface for urate crystal deposition. The big-toe MTP is a frequent site of osteoarthritis.
- Local fluid and pH changes: Overnight dehydration, cooling, and changes in synovial fluid volume or acidity can transiently increase urate concentration and favor precipitation.
- Crystal deposits are mechanically concentrated: Deposits tend to occur at the dorsal and medial parts of the 1st MTP, which are pressure-stressed during gait.
So, the classic location results from a combination of cold temperature + repeated trauma/loading + susceptible cartilage + local synovial conditions, not simply because serum uric acid is highest in the toe. Textbooks note that the 1st MTP is a classic focus, but gout can involve the ankle, midfoot, knee, wrist, fingers, and elbow as well. Review of podagra pathogenesis and a systematic review of the 1st MTP in gout describe this as multifactorial.
orotic aciduria can occur in ornithine transcarbomylase deficiency
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ornithine transcarbamylase deficiency orotic aciduria
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ornithine transcarbamylase deficiency orotic aciduria mechanism carbamoyl phosphate pyrimidine synthesis
Yes. Ornithine transcarbamylase (OTC) deficiency causes orotic aciduria.
Mechanism:
- OTC normally converts carbamoyl phosphate + ornithine → citrulline in mitochondria.
- With OTC deficiency, carbamoyl phosphate accumulates in mitochondria.
- Excess carbamoyl phosphate escapes into the cytosol.
- It enters the pyrimidine synthesis pathway, leading to excess orotic acid formation and urinary excretion.
Typical findings in OTC deficiency:
- Hyperammonemia
- Increased urinary orotic acid
- Low citrulline
- Low arginine
- Respiratory alkalosis early in acute hyperammonemia
- X-linked inheritance
Important comparison:
| Disorder | Ammonia | Orotic acid | Key point |
|---|---|---|---|
| OTC deficiency | High | High | Excess mitochondrial carbamoyl phosphate is diverted into pyrimidine synthesis |
| Carbamoyl phosphate synthetase I deficiency | High | Normal/low | Carbamoyl phosphate is not made, so none is diverted |
| UMP synthase deficiency | Normal | High | Causes megaloblastic anemia, but no hyperammonemia |
Harrison's notes that among proximal urea-cycle defects, urinary orotic acid is increased specifically in OTC deficiency. NCBI urea-cycle overview explains the cytosolic diversion of carbamoyl phosphate.
what is orotic aciduria
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orotic aciduria UMPS deficiency
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orotic aciduria UMP synthase deficiency clinical features treatment uridine
Orotic aciduria means an abnormally high amount of orotic acid in the urine. Orotic acid is an intermediate in the de novo pyrimidine synthesis pathway, used to make UMP and ultimately DNA/RNA nucleotides.
The term can refer to either a finding or a specific inherited disorder.
1. Hereditary orotic aciduria
This is a rare autosomal-recessive UMPS deficiency. UMP synthase normally converts orotic acid into UMP. When it is deficient:
- Orotic acid accumulates and is excreted in urine
- UMP and other pyrimidines are deficient
- Features include megaloblastic anemia that does not respond to folate or vitamin B12, poor growth/failure to thrive, and developmental delay
- Blood ammonia is normal
- Treatment is uridine supplementation, which bypasses the metabolic block and reduces new orotic acid production
2. Orotic aciduria in OTC deficiency
Here, the primary problem is a urea-cycle defect, not UMPS deficiency:
- OTC deficiency causes excess mitochondrial carbamoyl phosphate
- It enters the cytosol and is diverted into pyrimidine synthesis
- This produces excess orotic acid in urine
- Unlike hereditary orotic aciduria, it produces hyperammonemia, often with low citrulline and low arginine
| Feature | UMPS deficiency | OTC deficiency |
|---|---|---|
| Main pathway affected | Pyrimidine synthesis | Urea cycle |
| Blood ammonia | Normal | High |
| Megaloblastic anemia | Present | Not typical |
| Urinary orotic acid | High | High |
| Key mechanism | Cannot convert orotic acid to UMP | Excess carbamoyl phosphate drives orotic acid formation |
So, orotic aciduria is not itself a diagnosis. It is a biochemical sign whose clinical meaning depends especially on whether ammonia is high. NCBI's overview of hereditary orotic aciduria identifies it as UMPS deficiency, whereas NCBI's urea-cycle review explains why OTC deficiency also raises urinary orotic acid.
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