An 80-year-old man falls and suffers a compound fracture of the femur. Which immediate action is most appropriate?

Correct Answer: C

Rationale: The correct answer is C: Carefully splint the leg as it is. This is the most appropriate immediate action because splinting the leg as it is will help stabilize the fracture and prevent further injury or damage to surrounding tissues. Moving or straightening the leg (option D) could worsen the fracture and cause more pain. Positioning him flat on his back (option A) may not provide enough support and could lead to additional complications. Applying a tourniquet (option B) is not necessary and could potentially cut off blood flow to the leg, causing more harm. Splinting the leg as it is will help maintain alignment and reduce the risk of further damage until proper medical care can be provided.

Correct Answer: D

Rationale: The correct answer is D, TH1 differentiation vs TH2 differentiation, as it is not a random event. TH1 and TH2 differentiation is determined by cytokine signals, not random chance. A, Genetic recombination of VDJ segments for TCR assembly, and B, Genetic recombination of VDJ segments for BCR assembly, involve random processes during immune cell development. C, CD4 vs CD8 T cell fate, is determined by random rearrangement of gene segments during T cell development. Thus, D is the only choice that does not involve a random event in immune cell development.

Correct Answer: A

Rationale: The correct answer is A because developing T cells initially express both CD4 and CD8 co-receptors in the thymus. This stage is crucial for T cell maturation and selection. Eventually, T cells undergo positive and negative selection processes to become either CD4+ helper T cells or CD8+ cytotoxic T cells based on their specificity and function. Choices B and C are incorrect because T cells initially express both CD4 and CD8. Choice D is also incorrect as T cells do express either CD4 or CD8 during their development in the thymus.

Correct Answer: D

Rationale: Step 1: HIV is a retrovirus because it contains RNA as its genetic material and uses the enzyme reverse transcriptase to convert its RNA into DNA. Step 2: HIV is an RNA virus because its genetic material is RNA. Step 3: HIV is a lentivirus which is a subgroup of retroviruses known for causing slow-progressing diseases. Therefore, the correct answer is D as all statements A, B, and C are true based on the characteristics of HIV.

Correct Answer: B

Rationale: The correct answer is B: Infectious diseases. The immune system's main function is to protect the body from foreign invaders such as bacteria, viruses, and parasites. It recognizes and eliminates these pathogens to prevent infections. Autoimmune diseases (A) occur when the immune system mistakenly attacks healthy cells. Neurological disorders (C) and cardiovascular diseases (D) are not primarily defended against by the immune system as they are not caused by external pathogens.