Which sexually transmitted disease is caused by the bacterium Treponema pallidum? MCQ with Answer and Explanation

Explanation:
Double circulation (pulmonary and systemic) with a four-chambered heart keeps oxygenated and deoxygenated blood separate, increasing efficiency. R correctly explains A.

Explanation:
Vitamin B₁₂ contains cobalt, hence the name cobalamin. It is involved in DNA synthesis, red blood cell maturation, and nervous system maintenance. Deficiency leads to pernicious anemia and neurological disorders.

Explanation:
The Blood-Brain Barrier (BBB) is a highly selective semipermeable border that protects the brain from many substances in the blood. It is primarily formed by the tight junctions between the endothelial cells of the brain's capillaries. These endothelial cells are closely associated with the foot processes of astrocytes (a type of glial cell), which induce and maintain the tight junctions, restricting the passage of large molecules, toxins, and pathogens into the brain tissue.