The human spinal cord extends from the: MCQ with Answer and Explanation

Explanation:
The cardiac conduction system ensures coordinated heartbeats. The sinoatrial (SA) node generates the electrical impulse, acting as the natural pacemaker. The impulse reaches the atrioventricular (AV) node, which delays it slightly to allow the atria to fully contract and empty into the ventricles. The impulse then travels down the Bundle of His to the Purkinje fibers, which rapidly distribute it throughout the ventricular myocardium, causing ventricular contraction. All statements are correct.

Explanation:
The inner surface of the small intestine is lined with finger-like projections called villi, which increase the surface area for absorption. The epithelial cells (enterocytes) covering these villi have their own microscopic, finger-like projections on their apical surface, called microvilli. Collectively, these microvilli form a 'brush border'. This further massively increases the surface area, maximizing the efficiency of nutrient absorption. Cilia are motile, and stereocilia are found in the inner ear and epididymis.

Explanation:
Lysosomes are membrane-bound vesicles packed with hydrolytic enzymes (proteases, lipases, nucleases) that digest macromolecules, old organelles, and engulfed pathogens. They maintain an acidic pH (about 5.0) for optimal enzyme activity. Peroxisomes detoxify peroxides, mitochondria generate ATP, chloroplasts perform photosynthesis.