During midfetal life, the primary source of blood cells is the:

Correct Answer: D

Rationale: The correct answer is D: Liver. During midfetal life, the liver is the primary source of blood cells. The rationale behind this is that the liver is the major site of hematopoiesis during this developmental stage, producing red blood cells, white blood cells, and platelets. The liver's hematopoietic function gradually declines as the bone marrow takes over this role postnatally. Explanation for why other choices are incorrect: A: Bone marrow - While bone marrow becomes the primary site of hematopoiesis in adults, during midfetal life, it is not the main source of blood cells. B: Spleen - The spleen plays a role in blood filtration and immune response, but it is not the primary source of blood cells during midfetal life. C: Lymph Nodes - Lymph nodes are important for immune function and lymphocyte production, but they do not serve as the primary source of blood cells during midfetal life.

Correct Answer: C

Rationale: The correct answer is C (70+). Chronic lymphoblastic leukemia typically affects older adults, with the median age of diagnosis around 70 years old. This is due to the gradual accumulation of genetic mutations over time. Older patients are more likely to develop this type of leukemia compared to younger individuals. Choices A, B, and D are incorrect because they suggest younger age groups, which are less commonly associated with chronic lymphoblastic leukemia. Additionally, younger individuals are more likely to be affected by acute lymphoblastic leukemia rather than the chronic form.

Correct Answer: B

Rationale: The correct answer is B: slow-reacting substance A (leukotrienes). In severe asthma, leukotrienes are potent inflammatory mediators that cause bronchoconstriction, mucus production, and airway inflammation. Antihistamines target histamine, which is not the primary mediator in asthma. Interleukin-2 is involved in immune response modulation, not asthma symptoms. Serotonin plays a role in smooth muscle contraction, but it is not a major player in asthma pathophysiology. Bradykinin is a potent vasodilator and increases vascular permeability, but it is not the primary mediator in asthma. Therefore, the lack of relief from antihistamines suggests that leukotrienes are likely the cause of the symptoms in this patient with severe asthma.

Correct Answer: B

Rationale: The correct answer is B: immunoglobulin heavy-chain class. After binding to its specific antigen, a B lymphocyte can undergo class switching to change the type of antibody it produces while maintaining the same antigen specificity. This process involves recombination events that switch the constant region of the immunoglobulin heavy chain, leading to the production of antibodies with different effector functions. Choices A, C, and D are incorrect because they do not accurately describe the process of class switching in B lymphocytes. Light-chain isotypes, variable region of the heavy chain, and constant region of the light chain do not undergo class switching.

Correct Answer: A

Rationale: The correct answer is A: systemic anaphylaxis. When a child disturbs a wasp nest and is stung repeatedly, they can develop an allergic reaction leading to systemic anaphylaxis. This is a severe, rapid-onset allergic reaction that can cause shock due to widespread release of inflammatory mediators. Symptoms include difficulty breathing, swelling, low blood pressure, and can progress rapidly. Summary of other choices: B: Serum sickness is a delayed hypersensitivity reaction to foreign proteins, not typically seen in acute situations like this. C: An Arthus reaction is a localized immune complex-mediated hypersensitivity, not likely to result in systemic shock. D: Cytotoxic hypersensitivity involves antibodies attacking cells, not the rapid multi-system response seen in anaphylaxis.