Arthritis: MDCAT Biology notes
Arthritis for MDCAT: inflammation of joints, osteoarthritis vs rheumatoid arthritis vs gout vs septic arthritis, causes, autoimmunity and key exam facts.
What is arthritis?
Arthritis means inflammation of one or more joints. It is a disorder of the skeletal system, not of muscles or internal organs. The typical symptoms are pain, swelling, stiffness and reduced movement of the affected joint. Several different diseases fall under this heading, and MDCAT questions usually ask you to match each type with its cause.
Main types of arthritis
| Type | Cause | What happens in the joint |
|---|---|---|
| Osteoarthritis | Ageing; wear and tear | Articular cartilage gradually degenerates and wears away, so bone surfaces rub together. It commonly affects weight-bearing joints such as knees and hips. |
| Rheumatoid arthritis | Autoimmune | The body's own immune system attacks the synovial membrane; it becomes inflamed and thickened, and the cartilage and bone may be damaged. Often affects small joints of the hands and feet on both sides. |
| Gouty arthritis (gout) | Metabolic | Excess uric acid in the blood is deposited as crystals in joints (often the big toe), causing painful inflammation. |
| Septic (infectious) arthritis | Bacteria (infection) | Microorganisms invade the joint cavity and cause acute inflammation. |
Osteoarthritis: the commonest form
Osteoarthritis is by far the most common type of arthritis. It is a degenerative disease linked with ageing: over the years the smooth articular cartilage at the ends of the bones thins and breaks down. Without its cushion, the joint becomes painful and stiff. Obesity and repeated stress on a joint add to the wear.
Rheumatoid arthritis: an autoimmune disease
In an autoimmune disorder the immune system fails to recognise the body's own tissue as "self" and attacks it. In rheumatoid arthritis the target is the synovial membrane. This is the only skeletal disorder in the usual MDCAT options that is autoimmune.
Distinguish from other skeletal disorders
- Disc slip (herniation): the intervertebral disc bulges out and may press on a nerve; not arthritis.
- Sciatica: pain and tingling along the sciatic nerve down the leg, often due to a slipped disc.
- Spondylosis: degeneration of the vertebral column; the vertebrae may fuse.
- Osteoporosis: bones become porous and brittle due to loss of bone mass; a bone disease, not joint inflammation.
- Rickets: soft bones in children due to deficiency of vitamin D (poor calcium deposition).
Common MDCAT traps
- "Autoimmune skeletal disorder" means rheumatoid arthritis, not spondylosis, sciatica or rickets.
- "Most common" and "due to ageing" both point to osteoarthritis, not rheumatoid arthritis or gout.
- Septic arthritis is caused by bacteria; do not confuse it with ageing or metabolic causes.
- Osteoporosis sounds similar to osteoarthritis but is loss of bone density, not joint inflammation.
- Arthritis affects joints; herniation and tingling in the legs describe disc slip and sciatica.
Quick revision
- Arthritis = inflammation of joints.
- Osteoarthritis: commonest; age-related wearing of articular cartilage.
- Rheumatoid arthritis: autoimmune attack on synovial membrane.
- Gout: uric acid crystals in joints.
- Septic arthritis: bacterial infection of a joint.