This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
A 12 yr old child presents with fever and cervical lymphadenopathy for last 36 hours. Oral examination shows a grey membrane on the right tonsil. What is the primary treatment? 
Select and explain. 
A. Penicillin IV for 10 days, B. 20000 -40000 units I/M equine derived antitoxin, C. 80,000-1,20,000units I/M equine derived antitoxin, D. Erythromycin to be given for 2 weeks
B: 20000 -40000 units I/M equine derived antitoxin
Answer: B: 20000 -40000 units I/M equine derived antitoxin 
Explanation: The primary treatment is 20000 -40000 units I/M equine derived antitoxin. Above mentioned clinical signs and symptoms gives a presumptive diagnosis of Diphtheria. Treatment of diphtheria: - Equine derived Antitoxin-which neutralizes the circulating diphtheria toxin. - Moderate cases with <3 days (pharyngeal or laryngeal): 20000 -40000 units I/M. - Serious cases with >= 3days disease: 80,000 -1,20,000 units I/V and I/M combined. - Penicillin Or Erythromycin to be given for 2 weeks for eradication of bacterium from throat Isolation of the patient for 48 hrs.