**Question:** Structure without any membrane is

A. Nucleus
B. Mitochondria
C. Ribosome
D. Lysosome

**Correct Answer:** A. Nucleus

**Core Concept:** A membrane is a biological barrier that surrounds or separates various cellular components, allowing selective transport of molecules in and out of the structure while maintaining its internal environment. The nucleus is the control center of a cell, housing genetic material and involved in cellular processes like DNA replication, transcription, and translation.

**Why the Correct Answer is Right:** The nucleus does not have a membrane as it is enclosed within the nuclear envelope, which is a double membrane made up of endoplasmic reticulum. This arrangement provides protection to the genetic information and maintains the proper environment for the nucleus's functions.

**Why Each Wrong Option is Incorrect:**

A. B. Mitochondria: Mitochondria are known as the "powerhouse of the cell" because they produce adenosine triphosphate (ATP) through cellular respiration. They have a double membrane structure, which is crucial for their functions like energy production and maintaining the oxidative phosphorylation process.

B. C. Ribosome: Ribosomes are the sites of protein synthesis in cells, containing ribosomal RNA (rRNA) and ribosomal proteins. They have a unique structure but not a membrane surrounding them.

C. D. Lysosome: Lysosomes are membrane-bound organelles primarily involved in cellular digestion and waste disposal. They contain various hydrolytic enzymes and are found in most eukaryotic cells.

**Clinical Pearl:** Understanding the various organelle structures and their functions is essential for medical students and practitioners as it relates to cellular processes, organelle identification, and disease diagnosis. For example, understanding the role of mitochondria in cellular respiration aids in differentiating between normal and dysfunctional mitochondria in various pathologies like Kearns-Sayre syndrome or mitochondrial myopathies.