A patient with severe asthma gets no relief from antihistamines. The symptoms are MOST likely to be caused by

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: 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.

Correct Answer: D

Rationale: The correct answer is D because toxic shock syndrome (TSS) is caused by superantigens, not A-B toxins. Superantigens directly activate a large number of T cells by binding to both the MHC class II protein and the T cell receptor simultaneously. This leads to massive cytokine release and the symptoms described. Choices A, B, and C are all accurate statements about the mechanism of action of superantigens in TSS. Choice D is incorrect because TSS toxins do not have an A-B subunit structure; they are superantigens that directly interact with T cells and do not enter cells to activate them.

Correct Answer: D

Rationale: The correct answer is D: enzymes released by polymorphonuclear cells. When immune complexes are deposited in the glomerular basement membrane, neutrophils (a type of polymorphonuclear cell) are recruited. These neutrophils release enzymes such as elastase and collagenase, which directly damage the membrane. Gamma interferon (choice A) is involved in immune response but does not directly damage the membrane. Phagocytosis (choice B) is the process of engulfing and digesting particles, not directly causing damage to the membrane. Cytotoxic T cells (choice C) target and kill infected or abnormal cells, but they are not directly involved in damaging the glomerular basement membrane in this scenario.

Correct Answer: C

Rationale: The correct answer is C because complement lyses cells by forming a membrane attack complex (MAC) which involves the insertion of complement proteins into the cell membrane, leading to cell lysis. Choices A, B, and D are incorrect as complement does not lyse cells through enzymatic digestion, activation of adenylate cyclase, or inhibition of elongation factor 2. Complement-mediated lysis specifically involves the formation of MAC to disrupt the target cell membrane.