Metastasis is a complex process. It is the migration of tumor cells to other parts of the body from the site where the cancer began to grow.

For the last two decades, it has been known that viruses can be causative agents of cancer. Cancer causing agents were first described in viruses infecting poultry farms.

The body cells will not divide unless they get outside signals that cause the cells to enter into the cell cycle but the cancer cells are capable of growing in the absence of these signals. They do not obey the normal trends of cell division.

Methotrexate works by inhibiting an enzyme known as dihydrofolic acid reductase,  which is important for transforming an inactive form of folic acid into the active form, which is necessary to make some of the building blocks needed for DNA production. It inhibits the synthesis of DNA, RNA and proteins.

Oncogenic viruses are those which cause cancer or malignancy. E.g. Cancer of the cervix (part of uterus) is mostly caused by Human Papilloma Virus (HPV).

The trapezius muscle is a large, superficial muscle present in the neck, shoulders and back. It does not assist respiration whether at rest or during exercise.

The sodium-glucose cotransport is the mechanism for intestinal glucose absorption. The sodium glucose symporter does not use ATP. It uses the concentration of Na⁺ to drive glucose across intestinal epithelial cells.

Respiratory bronchioles are the smallest subdivision of a bronchiole, which connects the terminal bronchiole to an alveolar duct. They are the first branching of the bronchial tree.

During expansion, i.e. during the act of inspiration,  the surfactant density decreases which makes the surface tension strong enough to make the alveoli collapse again. So, the surface tension will constantly rise not as the option states that it will increase, then decrease.

Systemic venous blood is the blood which has given up its oxygen to the tissues and is carrying carbon dioxide back through the systemic veins for gaseous exchange in the lungs. So, the partial pressure of carbon dioxide will be the highest in systemic venous blood.

Special chemical receptors close to the aorta and carotid arteries are highly responsive to an increase in carbon dioxide or acid concentration, or to a decrease in the pressure of oxygen (PaO₂). When these receptors sense acidity or low oxygen, they stimulate the brain respiratory centre to increase the speed and depth of breathing.

A proto-oncogene is a normal body gene, which becomes an oncogene due to mutations or increased expression. They code for proteins controlling cell growth and differentiation.

Tumor vasculature originates through the process of angiogenesis in which existing blood vessels are recruited and remodelled to provide increased hyper blood flow to avascular tumor tissues.

This statement is incorrect. In respiratory distress syndrome, the lungs alveoli are collapsed not expanded. The expanded alveoli, which sometimes burst is a signature characteristic of emphysema, not the respiratory distress syndrome.

The pneumotaxic centre is a network of neurons in the rostral dorsal lateral pons. Its main function is to inhibit inspiration and thus regulating inspiratory volume and rate.

Stagnant hypoxia occurs when something obstructs the flow of the blood carrying an adequate amount of oxygen. The major causes include heart attack, pulmonary embolism, chronic alveolar hypoventilation and choking.

Alveolar ventilation gives the oxygen necessary for oxidative metabolism and elucidates the CO₂ produced by these metabolic processes. Therefore, it is appropriate that the main physiologic stimuli to respiration are an elevation in the PCO₂ and a reduction in the PO₂ (hypoxemia).

It is the state in which a reduced amount of air enters the lungs alveoli resulting in decreased levels of oxygen and increased levels of carbon dioxide of the blood. Thus, the case dissused above is a case of hypoventilation.

The diaphragm alone can maintain adequate ventilation at rest. The muscles aid the flattened diaphragm in generating sufficient inspiratory pressure to maintain adequate ventilation, both when awake and during rest.

The stagnant hypoxia is defined as a condition in which there is something obstructing the blood flow even though the blood has an adequate amount of oxygen. Thus, a sudden shock can cause stagnant hypoxia in an individual.

When the hemoglobin level is slightly below normal in an individual,  then the feeling of tiredness, headache, dizziness, fatigue and lack of concentration are common. Physically, the visible signs are pale skin with bluish colour of the skin and mucous membranes, nail beds and gums.