This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Mycobacterial Infections, Subject name: Dental
Patient with leprosy, smear sample taken show 10- 100 bacilli in one field. Bacterial index is: 
Select and explain. 
A. 1+, B. 2+, C. 3+, D. 4+
D: 4+
Answer: D: 4+ 
Explanation: Bacterial index is calculated by counting the number of bacilli in 100 fields under oil immersion Grade Number of bacilli 1+ 1 to 10 bacilli per 100 fields 2+ 1 to 10 bacilli per 10 fields 3+ 1 to 10 bacilli per field 4+ 10 to 100 bacilli per field 5+ 100 to 1000 bacilli per field 6+ Multiple clumps of bacilli present