This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Integrated QBank, Subject name: Unknown
A 80-year-old male presented with acute shoness of breath. His SpO2 levels were 80%. He was immediately intubated and shifted to the ICU and put on the ventilator. He was a chronic smoker and a known case of COPD for the past 5 years. He was staed in empirical antibiotics. Initial culture tests were negative. After 9 days, he developed ceain lesions in the lungs along with high grade fever. A bronchoalveolar lavage was done and sent for gram staining. The organism isolated was a multi-drug resistant organism. Biochemical reactions revealed oxidase positive and did not ferment any sugars. The organism isolated is resistant to all antibiotics except: - 
Select and explain. 
A. Colistin, B. Amoxicillin, C. Azithromycin, D. Amikacin
A: Colistin
Answer: A: Colistin 
Explanation: This is a case of Acinetobacter baumanii. X-ray shows consolidation patches in both lungs. Gram stain shows gram-negative coccobacilli- suggestive of Acinetobacter baumanii. A. baumannii is a notorious cause of nosocomial pneumonia Most frequently among patients requiring prolonged mechanical ventilation. It is resistant to all antibiotics except carbapenems and colistin.