Renal stone or calculus or lithiasis is one of the most common diseases of the urinary tract. It occurs more frequently in men than in women and in whites than in blacks. It is rare in children. It shows a familial predisposition.
Urinary calculus is a stone-like body composed of urinary salts bound together by a colloid matrix of organic materials. It consists of a nucleus around which concentric layers of urinary salts are deposited.

Aetiology

1. Hyperexcretion of relatively insoluble urinary constituents such as oxalates, calcium, uric acid, cystine and certain drugs (such as magnesium trisilicate in the treatment of peptic ulcer).

2. Physiological changes in urine such as Urinary pH (which is influenced by diet and medicines), Colloid content, Decreased concentration of crystalloids, Urinary magnesium/calcium ratio.

3. ALTERED URINARY CRYSTALLOIDS AND COLLOIDS
· Either there is an increase in the crystalloid level or a fall in the colloid level, urinary stones may be formed.
· If there is any modification of the colloids e. g. they lose their solvent action or adhesive property, urinary stones may develop.

4. HYPERPARATHYROIDISM : Due to overproduction of parathormone the bones become decalcified and calcium concentration in the urine is increased. This extra calcium may be deposited in the renal tubules or in the pelvis to form renal calculus.

5. URINARY STASIS : It causes a shift of the pH of the urine to the alkaline side, predisposes urinary infection, and allows the crystalloids to precipitate.

6. URINARY INFECTION : Infection disturbs the colloid content of the urine, also causes abnormality in the colloids (which may cause the crystalloid to be precipitated). Infection also changes urinary pH and also causes increase in concentration of crystalloids.