5. There are many cases of human disease in which one or another enzyme activity is lacking due to genetic mutation. However, cases in which individuals lack one of the enzymes of the citric acid cycle are extremely rare. Why

The citric acid cycle is fundamental to produce the energy needed for aerobic cell metabolism, thereby mutations in the enzymes involved in this metabolic pathway would be lethal to the individual

Explanation:

In aerobic organisms, the citric acid cycle (also known as the Krebs cycle), is a key metabolic pathway used to remove electrons and uses them in the electron transport chain in order to generate adenosine triphosphate (ATP), the energy currency of the cell. The citric acid cycle depends on eight enzymes that oxidize acetyl-CoA into two molecules each of carbon dioxide (CO2) and water (H2O). Thus, the citric acid cycle is a series of chemical reactions that are key to breakdown carbohydrates, fats and proteins into CO2 and H2O in order to produce energy (ATP).

thrombocytopenia, which is a deficiency in blood platelets, can cause bleeding and bruising in the dermis similar to senile purpura. other possible causes include: exposure to ultraviolet rays and aging skin are common causes of senile purpura. senile purpura may also be a sign of collagen loss in the skin and bones.

the unequal cytoplasmic division that  results  in  one egg  and three polar bodies ensures that a fertilized  egg has  ample nutrients  forits journey  to  the uterus.