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: Microbiology
Which of the following is always present in a virus? 
Select and explain. 
A. Enzymes, B. Envelop, C. RNA or DNA, D. All of the above
C: RNA or DNA
Answer: C: RNA or DNA 
Explanation: Ans. is 'c' i.e., RNA or DNA Viruses are obligate intracellular parasites.They lack enzymes necessary for protein and nucleic acid synthesis and are dependent for replication on the synthetic machinery of host cells - so, they cannot grow in cell free culture media.They do not have cellular organization - do not contain ribosome, ER, mitochondria etc.They contain only one type of nucleic acid, either RNA or DNA, never both.Virion may be enveloped or nonenveloped (naked).