This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
All of the following form radiolucent stones except: 
Select and explain. 
A. Xanthine, B. Cysteine, C. Allopurinol, D. Orotic acid
B: Cysteine
Answer: B: Cysteine 
Explanation: Plain radiograph 
Calcium-containing stones are radiopaque: 

Calcium oxalate +/- calcium phosphate 
Struvite (triple phosphate) - usually opaque, but variable 
Pure calcium phosphate 

Lucent stones include: 

Uric acid 
Cystine
Medication (indinavir is best known) stones 
Pure matrix stones (although may have a radiodense rim or centre) 

Key Point:
Cysteine and Cystine are different amino acids.