This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary tract, Subject name: Pathology
Birefringent crystal in urine is seen with 
Select and explain. 
A. Calcium oxalate stones, B. Uric acid stones, C. Struvite stones, D. None
A: Calcium oxalate stones
Answer: A: Calcium oxalate stones 
Explanation: There are two types of calcium oxalate crystals, the dihydrate (or Wedellite) and the monohydrate (orWhewellite), which are frequently found together inthe same sample. The former have mostly a typical bipyramidal shape, while the latter are more pleiomor- phichic , although the ovoid shape is the most frequent. Bipyramidal crystals are birefringent only when large or in aggregates, but even then birefringence is usually not intense. The monohydrates, however,are always strongly birefringent (Table 1). Calcium oxalate may be found in normal subjects, often as a consequence of ingestion of foods like chocolate, beet-root, peanuts, rhubarb, spinach, etc. , in stone formers, in patients with hyperoxaluria, or after ethylene glycol poisoning (see 'Crystalluria and acute renal failure'). Refer robbins 9/e p951