This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urethra & Penis, Subject name: Surgery
What characteristic of struvite (ammonium magnesium phosphate) stones makes antibiotics ineffective when treatment is being performed? 
Select and explain. 
A. Resistant bacteria, B. Poor excretion of antibiotics, C. Ineffective antibiotics, D. Bacteria inaccessible to antibiotics
D: Bacteria inaccessible to antibiotics
Answer: D: Bacteria inaccessible to antibiotics 
Explanation: Bacteria inaccessible to antibiotics. Struvite calculi harbor infective bacteria within their interstices. Effective therapy must be directed to eradicate associated infection. Struvite stones are the second most common type of renal calculi after calcium oxalate stone.