Which function is specific to B lymphocytes?

Choice A rationale

Increased protein synthesis is typically an anabolic process that occurs under the influence of growth hormone or testosterone, and it is not a hallmark of hypothyroidism. In a hypothyroid state, there is actually a decrease in both protein synthesis and protein degradation. The lack of thyroid hormone leads to a general slowing of all cellular activities. Weight gain in this condition is not due to the accumulation of muscle mass or protein but rather metabolic slowdown.

Choice B rationale

Thyroid hormones, specifically thyroxine and triiodothyronine, are the primary regulators of the basal metabolic rate, which is the amount of energy the body expends at rest. In hypothyroidism, low levels of these hormones lead to a significant decrease in oxygen consumption and caloric burning by cells. Consequently, even with a normal or reduced caloric intake, the body stores more energy as fat and retains excess water and mucopolysaccharides, leading to the characteristic weight gain.

Choice C rationale

Increased glucose metabolism would typically lead to weight loss or increased energy expenditure, which is seen in hyperthyroidism, not hypothyroidism. In a hypothyroid state, the rate of glucose absorption from the gastrointestinal tract is decreased, and the peripheral uptake of glucose by cells is slowed down. The body's inability to efficiently process and burn glucose as fuel contributes to the overall sluggishness and weight gain associated with the deficiency of circulating thyroid hormones.

Choice D rationale

Thyroid hormones normally increase the sensitivity of cells to catecholamines like epinephrine and norepinephrine by upregulating beta-adrenergic receptors. In hypothyroidism, there is a decreased sensitivity to these catecholamines. This reduction in sympathetic nervous system responsiveness leads to a slower heart rate and decreased lipolysis, which is the breakdown of fats. The inability to effectively mobilize fat stores for energy due to this decreased sensitivity contributes directly to the accumulation of adipose tissue.

Choice A rationale

Giving an incompatible blood type never results in increased oxygen delivery to the tissues. Instead, the massive destruction of red blood cells significantly reduces the oxygen-carrying capacity of the blood. The resulting acute hemolytic transfusion reaction causes systemic instability and potential circulatory collapse. Oxygenation is further compromised by the formation of microemboli and the potential for acute lung injury, which are common complications of major ABO incompatibility errors.

Choice B rationale

The administration of incompatible blood triggers a massive, hyperactive immune response rather than suppression. The recipient's pre-existing antibodies immediately identify the foreign antigens on the donor cells, initiating a cascade of complement activation and inflammatory cytokine release. This leads to systemic symptoms such as fever, chills, and hypotension. Suppression of the immune system is generally an intentional therapeutic goal achieved through pharmacology, not a consequence of a mismatched blood transfusion.

Choice C rationale

A reaction is certain when type A blood receives type B blood because the recipient possesses naturally occurring anti-B antibodies. These antibodies are present in the plasma of any individual with type A blood and will recognize the B antigens on the donor erythrocytes. The interaction between the antigen and antibody is immediate and severe, making a "no reaction" scenario biologically impossible in a person with a functioning and healthy immune system.

Choice D rationale

When type B blood enters the circulation of a type A recipient, the anti-B antibodies in the recipient's plasma bind to the B antigens on the donor red blood cells. This causes agglutination, or clumping, of the cells. Following agglutination, the complement system is activated, leading to intravascular hemolysis where the red cells are ruptured. This releases hemoglobin into the plasma, which can cause acute renal failure and disseminated intravascular coagulation.