This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cell Physiology and Intercellular Connections, Subject name: Physiology
Structure without any membrane is? 
Select and explain. 
A. Mitochondria, B. Nucleus, C. Nucleolus, D. Endoplasmic reticulum
C: Nucleolus
Answer: C: Nucleolus 
Explanation: ANSWER: (C) NucleolusREF: Guyton's 11th ed ch:2Most organelles of the cell are covered by membranes composed primarily of lipids and proteins.These membranes include the cell membrane, nuclear membrane, membrane of the endoplasmic reticulum, and membranes ofthe mitochondria, lysosomes and Golgi apparatus.The nuclei of most cells contain one or more highly staining structures called nucleoli. The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accumulation of large amounts of RNA and proteins of the types found in ribosomes. The nucleolus becomes considerably enlarged when the cell is actively synthesizing proteins.