1 I L Brown: MCQs in Pathology This edition (1985) derives its questions from the 12th Edition of Muirs Textbook of Pathology. Contents 1. Genetics and Disease 2. Cell Damage 3. Inflammation 4. Healing and Repair 5. The Immune response 6. Immunopathology 7. Infection 8. Disturbances of Blood Flow 9. Tissue Degenerations 10. Tumours: General Features 11. Blood Vessels 12. Heart 13. Respiratory System 14. Haemopoietic System 15. Lympho-Reticular System 16. Alimentary Tract 17. Liver, Biliary Tract, Pancreas 18. Nervous System 19. Urinary System 20. Locomotor System 21. Female Reproductive System 22. Male Reproductive System 23. Endocrine System 24. Skin 25. Parasitic Diseases 1. Genetics and Disease 1. For each of the features of DNA replication listed on the left select the most appropriate association from the list on the right. a. Splicing b. Transcription c. Translation A. Enzyme induced methylation. B. mRNA copy of DNA. C. Production of polypeptide chains from tRNA. D. Removal of superfluous introns. E. Stretches of DNA nearer 3' end of chain. 2 1. The answer is D, B, C. During splicing superfluous information transcribed from the introns of the genes is edited out of the mRNA. Transcription is the process whereby an mRNA copy of the DNA of the gene is produced before a polypeptide can be synthesised. Translation comprises production of tRNA by mRNA, with the tRNA amino acids arranged in the order necessary for the production of the specified polypeptide. Stretches of DNA nearer 3' end of chain are promoters which control transcription by acting as switches upstream to the gene to be transcribed. Enzyme induced methylation is a post-translation modification of polypeptide in the somatic cell. 2. For each of the patterns of inheritance on the left select the most appropriate disease from those on the right. a. Autosomal recessive. b. Dominant. c. X-linked recessive. A. Ankylosing spondylitis. B. Breast cancer. C. Duchenne muscular dystrophy. D. Glycogen storage disease. E. Neurofibromatosis. The answer is D, E, C. Glycogen storage disease is an autosomal recessive specific enzyme deficiency that results in excessive accumulation of glycogen in liver, kidney, myocardium. In neurofibromatosis a dominant character is expressed in one parent and can be expected in half of the offspring; multiple tumours of small nerves are present in this condition. Duchenne muscular dystrophy is X-linked recessive disease. The gene for this form of muscular dystrophy is present on the X chromosome, hence females are not affected (being XX) while males are (being XY). Ankylosing spondylitis is a disease of the vertebral column which is associated with a specific HLA type. Breast cancer may tend to recur in particular families but there is no known genetic component. 2. Cell Damage 3 3. For each of the causes of membrane damage listed on the left select the most appropriate association from those listed on the right. a. C. welchii aphlatoxin. b. Hypoxia. c. Ouabain. A. Digestive damage to cell membrane. B. Failure of oxidative phosphorylation in mitochondria. C. Mechanical rupture of cell membrane. D. Poisoning of cell membrane ion pump. E. Stimulation of cell membrane ion pump. The answer is A, B, D. Clostridium welchii aphlatoxin is a lecithinase which digests cell membrane resulting in increased permeability. Hypoxia results in decreased oxidative phosphorylation in mitochondria with resultant lack of ATP. Ouabain is a specific cell membrane ion pump antagonist which competes with K+ for a site on Na+K+ dependent ATP ase. 4. For each of the features on the left select the most appropriate association from the list on the right. a. Differentiation. b. Heterotopia. c. Metaplasia. A. Development of a clone of cells growing independently of normal cellular control. B. Loss of cellular specialization. C. Result of error in intercellular communication in developing fetus. D. Result of gene activation due to environmental change. E. Structural and functional specialization of cells. The answer is E, C, D. Differentiation is a structural and functional specialization of cells. This is a feature of multicellular organisms, and depends on the selection of genes which each cell type expresses. Heterotopia is a result of error in intercellular communication in developing fetus. Groups of cell differentiate in a way which is inappropriate to their anatomical location. Metaplasia is a result of gene activation due to environmental change. This is the change of one differentiated cell type into another, and is often seen as a result of environmental factors, i.e., cigarette smoking. Loss of cellular specialization is dedifferentiation. 4 Neoplasia comprises the development of a clone of cells growing independently of normal cellular controls. 5. For each example of necrosis listed on the left select the most suitable description from those on the right. a. Caseous necrosis. b. Coagulative necrosis. c. Colliquative necrosis. A. Cheese-like material. B. Dull, swollen firm area. C. Firm yellow/white patches. D. Green/black discolouration. E. Soft, liquefying material. The answer is A, B, E. Caseous necrosis - cheese-like material. This is the typical appearance of tuberculous necrosis, but a similar appearance may also be seen in necrotic tumours, particularly squamous carcinoma. Coagulative necrosis - dull, swollen firm area. This is typical of infarction in solid organs such as heart, kidney. Histologically 'ghost' outlines of the tissue structure are still present. Colliquative necrosis - soft, liquefying material. This is typical of necrotic tissue with a high content of fluid, i.e., brain tissue. Histologically the tissue structure is lost. 3. Inflammation 6. If the following features of the acute inflammatory reaction were placed in chronological order which would come fourth? A. Arteriolar contraction. B. Blood flow slows. C. Dilatation of arterioles. D. Emigration of leucocytes from blood vessels. E. Protein rich fluid escapes from blood vessels. The answer is B. In the acute inflammatory response the injury results in an initial contraction of arterioles (A) followed rapidly by arteriolar dilatation (C) in the process of active hyperaemia; as a result of the hyperaemia the inflammatory exudate is formed (E) and is responsible for swelling and pain; the microcirculation remains engorged, but blood flow slows down (B) with associated emigration of leucocytes (D). 7. Which ONE of the following ultrastructural features is believed to allow for the increased permeability of the vascular endothelium in acutely inflamed tissue? A. Cytoplasmic micropinocytotic vesicles. 5 B. Gaps in endothelial tight junctions. C. Gaps in the basement membrane. D. Increase in number of phagolysosomes. E. No morphological changes. The answer is B. There is experimental evidence that gaps appear between vascular endothelial cells during acute inflammation caused by injury and by chemical mediators. These gaps are temporary. Micropinocytotic vesicles (A) do transfer material across cells but are not increased in inflammation. 8. For each of the phases of increased vascular permeability in the acute inflammatory reaction noted on the left choose the most suitable association from those on the right. a. Immediate sustained response. b. Immediate transient response. c. Delayed prolonged leakage. A. Endothelial cells elongate. B. Leakage occurs through vascular endothelium. C. Leakage occurs through venules and capillaries. D. Secretion of exogenous mediators by endothelial cells. E. Surrounding tissue and endothelium damaged. The answer is E, B, C. Immediate sustained response - surrounding tissue and endothelium damaged. This occurs in more severe injury in which vascular damage may be so great as to cause thrombosis and even infarction of the tissues. Immediate transient response - Leakage occurs through vascular endothelium. In the experimental models only venular leakage occurs in this phase; this suggests endogenous mediator activity. Delayed prolonged leakage - Leakage occurs through venules and capillaries. Both capillaries and venules leak in this phase, but leakage is confined to the zone of injury suggesting that this is due to direct endothelial injury. 9. Which ONE of the following is not an endogenous mediator of increased vascular permeability? A. Angiotensin. B. C3a and C5a. C. 5-hydroxytryptamine. D. Kallikrein. E. Prostaglandin E2. The answer is A. Angiotensin is produced by the action of renin on angiotensinogen and is involved in the secretion of aldosterone and in pressor effects. 6 C3a and C5a (B) are part of the complement cascade and are activated C3 and C5; they act by liberating histamine from mast cells. 5-hydroxytryptamine (C) or serotonin causes increased vascular permeability in rodents but not in man. Kallikrein (D) is produced by the activation of Hageman factor (factor XII) producing prekallikrein activator which converts prekallikrein to kallikrein. Prostaglandin E2 (E) is secreted by polymorphs which are phagocytically active, it does not cause increased permeability itself but potentiates the activity of other factors. 10. Which ONE of the following is not a useful effect of acute inflammation? A. Dilution of toxins. B. Formation of fibrin. C. Phagocytosis. D. Stimulation of immune response. E. Swelling of tissues. The answer is E. Tissue swelling may result in obstruction of a vital passageway, i.e., larynx, or may cause ischaemic necrosis within an enclosed space, i.e., testis. The others are all useful but some people may have an inappropriate immune response and may therefore develop a pathological condition as part of their physiological response, i.e., asthmatics. There is also a rare condition in which a deficiency of a complement activation controlling factor (C1-inhibitor) allows complement activation to occur (angio- neurotic oedema). 11. Which ONE of the following is not an acceptable characteristic of a granuloma. A. Composed of altered macrophages. B. Composed of fused macrophages (giant cells). C. Composed of epithelioid cells. D. Composed of a mixture of chronic inflammatory cells. E. Composed of polymorphonuclear leucocytes, cellular debris and fibrin. The answer is E. Composed of polymorphonuclear leucocytes, cellular debris and fibrin - This is a description of pus as would be found in an abscess. Polymorphonuclear leucocytes and nuclear debris can be found in a true granuloma if there is a focus of suppuration: an infective granuloma. The definition of 'granuloma' is controversial; it may be used to mean a chronic inflammatory lesion forming a tissue mass or it may be restricted to a lesion composed of macrophages or even of altered macrophages (epithelioid cells). 12. For each of the cell type listed on the left choose the most appropriate association from those on the right. 7 a. Alveolar macrophages. b. Kupffer cells. c. Langhans' giant cells. A. CNS phagocytes. B. Digest bone matrix. C. Lining cells of hepatic sinusoids. D. Nuclei arranged peripherally in the cytoplasm. E. Phagocytic activity dependent on oxygen. The answer is E, C, D. Alveolar macrophages - phagocytic activity dependent on oxygen. The alveolar macrophages illustrate the point that local environment may influence cellular function; unlike other phagocytic cells these cells require high oxygen tension for full activity. Kupffer cells - lining cells of hepatic sinusoids. The Kupffer cell lines the hepatic sinusoids and is active in phagocytosis of particulate matter in the liver. Langhans' giant cell - nuclei arranged peripherally in the cytoplasm. The Langhans' giant cell characteristically has a large number of peripherally arranged nuclei; this cell is typical of the tuberculous granuloma. Central nervous system phagocytes. The representative of the mononuclear phagocyte system in the CNS (A) is the microglial cell. Digest bone matrix. Osteoclasts are derived from bone marrow precursors and digest matrix (B). 4. Healing and Repair 13. If the following events were placed in their most probable order of occurrence following a skin wound which would come fourth? A. Blood clot formation. B. Growth of granulation tissue. C. Loss of vascularity of fibrous scar. D. Migration of epithelial cells from wound edges. E. Wound edges united by collagen. The answer is E. In a wound healing by first intention the gap fills with blood clot (A); epithelium migrates from the wound edges and within 48 hours the gap is bridged by epithelium (D); granulation tissue grows into the wound (B) forming a scaffolding by which the wound edges are united by collagen (D), producing a narrow fibrous scar which gradually loses its vascularity (C). 14. From the list on the right choose the most suitable association for each of the types of collagen listed on the left. 8 a. Type I collagen. b. Type II collagen. c. Type IV collagen. A. Basement membrane. B. Cartilage. C. Dermis. D. Embryonic dermis. E. Synovial membrane. The answer is C, B, A. Type I collagen is found in dermis, tendon, bone, dentine and cornea. (C) Type II collagen is found in cartilage, intervertebral disks, vitreous body. (B) Type III collagen is found in embryonic dermis, early scar tissue, synovial membrane and is known as reticulin. Type IV collagen is found in basement membrane and shows a different ultrastructural appearance from the others (it does not have the typical 64 nm banding). (A) 15. If the following events in the healing of an open wound were in their most probable order which would come fourth? A. Emigration of polymorphonuclear leucocytes and macrophages. B. Epithelial proliferation. C. Myofibroblast contraction. D. Orientation of fibroblasts parallel to the capillary buds. E. Proliferation of new capillaries from the base of the wound. The answer is D. In a wound healing by second intention (either an open wound or ulcer with loss of tissue, or an infected wound) there is emigration of polymorphonuclear leucocytes, macrophages from the vessels (A) initially with the presence of more abundant fibrinous exudate. Epithelial proliferation (B) is the first sign of healing followed by the proliferation of capillary buds (E) forming granulation tissue with the associated proliferating fibroblasts (D). The fibroblast orientation later becomes parallel to the epithelial surface and contractility of these 'myofibroblasts' (C) helps to reduce the area of the open wound. Scar tissue is usually more prominent than following first intention healing. 16. Which ONE of the following does not impair healing of a wound? A. Deficiency of galactosamine. B. Deficiency of vitamin C. C. Excess of adrenal glucocorticoid hormones. D. Good vascular supply. E. Tissue hypoxia. 9 The answer is D. Wounds in areas of poor vascularity (i.e., the skin of the shin) heal very slowly compared with wounds of the face and scalp. Tissue hypoxia following severe injury results in poor healing (E). Galactosamine and vitamin C (A, B) are essential for the repair of ground substances and collagen including type IV (basement membrane) collagen. Excessive glucocorticoids (C) are associated with poor healing, and this may be seen in patients on long-term steroid therapy. 17. Which of the following events occurs fourth in sequence after a fracture of a typical long bone? A. Capillary proliferation from viable marrow. B. Increased osteoclastic resorption. C. Lamellar bone replaces woven bone. D. Ossification of the persistent fibrin clot. E. Periosteal proliferation around the fractured bone ends. The answer is D. Following fracture of a long bone provisional callus is formed by the proliferation of periosteal inner layer (E) which forms a cuff of bone trabeculae which produce the external callus; medullary cavity reaction (A) results in organization of the fibrin clot with production of woven bone in the marrow spaces; cortical reaction (B) results in increased osteoclastic resorption. The external callus unites the fragments externally, but not the bone ends which are joined by the fibrin clot and debris which in turn are ossified by osteogenic cells from the medullary cavity and periosteal callus (D). The final steps is remodelling of the bone (C) with formation of lamellar bone and resorption of the external callus, and eventually medullary callus. 5. The Immune Response 18. For each of the features of the immunoglobulin molecule listed on the left select the most appropriate association from those on the right. a. Fab fragment. b. Fc fragment. c. F(ab1)2 fragment. A. C-terminal region. B. Consists of heavy chains only. C. Consists of light chain and part of heavy chain. D. Consists of light chains only. E. Pepsin digestion product. The answer is C, A, E. Fab fragment - consists of light chain and part of heavy chain. Papain digestion of monomeric immunoglobulin results in the production of two antibody- binding fragments (Fab) which consist of light chains plus part of the heavy chain. Fc fragment - C-terminal region. Papain digestion of monomeric immunoglobulin results in the production of a fragment consisting of the C-terminal ends of the heavy chains linked together (Fc). 10 F(ab1)2 fragment - pepsin digestion product. Pepsin digestion splits the immunoglobulin molecule to produce a fragment consisting of two Fab fragments united by a portion of the Fc fragment. 19. For each of the types of immunoglobulin listed on the left select the most appropriate association from those on the right. a. IgG class specific antibody. b. IgM class specific antibody. c. Dimeric IgA class antibody. A. J-chain. B. Lymphocyte surface antigen receptor. C. Mast cell degranulation. D. Primary antibody response. E. Secondary antibody response. The answer is E, D, A. Following injection of antigen into an animal not previously exposed to that antigen there is a transient appearance in the blood of a small quantity of specific IgM class antibody in about 7 days (primary antibody response); re-injection of the antigen at a later date results in production of IgG class antibody in large amounts within 4 days (secondary antibody response). IgA is secreted by plasma cells as a dimer, i.e., 2 molecules linked together by a polypeptide, J-chain. IgM is also produced as a pentamer, the 5 IgM molecules being linked by 1 J-chain and 4 disulphide bonds. Mast cell degranulation (C) is a property of IgE. Lymphocyte surface antigen receptor (B) is a property of IgD. 20. For each of the types of T lymphocyte listed on the left select the most appropriate association from the list of surface markers on the right. a. Helper T lymphocytes. b. Prothymocytes. c. Suppressor T lymphocytes. A. T4 positive cells. B. T4 - T8 positive cells. C. T8 positive cells. D. T9 positive cells. E. T10 positive cells. The answer is A, E, C. Monoclonal antibodies to T lymphocyte surface antigens have resulted in identification of various subtypes of T lymphocyte. T9 positive cells are primitive T lymphocytes. Cells bearing both T4 and T8 are common thymocytes. 11 Helper T lymphocytes - T4 positive cells. Prothymocytes - T10 positive cells. Suppressor T lymphocytes - T8 positive cells. 21. If the following features were put in chronological order which would come fourth? A. Exposure to antigen. B. Differentiation into plasmablasts. C. Immunoblast proliferation. D. Production of mature plasma cells. E. Stimulation of germinal centre cells. The answer is B. Exposure to antigen (A) results in stimulation of (E) the cells in the superficial cortex of the lymph node (germinal centre cells); these cells enlarge and develop into B-immunoblasts (C) which divide and some differentiate into plasmablasts (B), which mature into plasma cells (D). A proportion of the B-immunoblasts differentiate into B memory cells and join the circulating pool of small lymphocytes. 22. For each of the features of a lymph node listed on the left select the most appropriate association from those on the right? a. Expansion of the deep cortex (paracortex) of the lymph node. b. Primary lymphoid nodules. c. Production of germinal centres. A. Macrophage predominates. B. Plasma cells predominate. C. Region of T-cell response. D. Superficial cortex of stimulated lymph node. E. Superficial cortex of unstimulated lymph node. The answer is C, E, D. Expansion of the deep cortex (paracortex) of the lymph node - region of T-cell response. The deep (or para) cortex is the T-lymphocytic zone of the lymph node and this enlarges during antigenic stimulus resulting in a cell mediated response. Primary lymphoid nodules - superficial cortex of unstimulated lymph node. In the unstimulated lymph node there are localized aggregates of lymphocytes in the superficial cortex. Production of germinal centres - superficial cortex of stimulated lymph node. Following antigenic stimulation of a type resulting in antibody production (humoral response) the primary nodules enlarge, to become germinal centres where B-lymphocytes proliferate. During the humoral response plasma cells are produced and these may be seen in the cortex deep to the germinal centres and in the medullary cords. 12 Macrophages are present lining the lymph sinuses and in some forms of stimulation may become very prominent (sinus histiocytosis). 6. Immunopathology 23. For each of the components of complement listed on the left select the most appropriate association from the features on the right. a. C3b. b. C5a. c. C5b-9 complex. A. Chemotactic for neutrophil polymorphs. B. Deficiency has no pathological effect. C. Inhibits mast cell degranulation. D. Macrophage surface receptor. E. Target cell plasma membrane injury. The answer is D, A, E. C3b - macrophage surface receptor. Macrophages (and polymorphs) have surface receptors for C3b which results in enhanced adherence of these cells to target cells with C3b on their surface. C5a - chemotactic for neutrophil polymorphs. C5a promotes the emigration and accumulation of neutrophil polymorphs and macrophages. C5b-9 complex - target cell plasma membrane injury. The final product of the complement cascade is a complex of C5b6789 which is inserted into the target cell plasma membrane resulting in cell lysis. Deficiency of the early stages of the complement cascade has no pathological effect. C3a and C5a both stimulate mast cell and basophil degranulation. 24. If the following features of the atopic reaction were placed in their correct order which would come fourth? A. Antigen absorbed for the second time. B. Degranulation of mast cells. C. Inhalation of pollen. D. Mast cell binding by Fc component of IgE. E. Production of IgE. The answer is A. Atopy (anaphylactic, immediate or type 1 hypersensitivity) occurs when IgE binds to mast cells and causes degranulation; antigen is absorbed (C) and the immune response produces specific IgE (E) which binds by its Fc component to mast cells (D); subsequent exposure to the antigen (A) results in antigen trapping by the IgE Fab components on the mast cells with subsequent degranulation (B). 13 25. For each of the hypersensitivity reactions on the left select the most appropriate association from the conditions on the right. a. Arthus reaction. b. Cytotoxic antibody reaction. c. Delayed hypersensitivity reaction. A. Asthma. B. Auto-immune haemolytic anaemia. C. Extrinsic allergic alveolitis. D. Infantile eczema. E. Tuberculoid leprosy. The answer is C, B, E. Artus reaction - extrinsic allergic alveolitis. Immune complex, Arthus type (type 3) reaction is the basis of extrinsic allergic alveolitis (farmer's lung) which is a reaction to bacterial spores growing on mouldy hay. Cytotoxic antibody reaction - auto-immune haemolytic anaemia. Cytotoxic antibody (type 2) reactions are mediated by antibody which combines with cell surface antigenic determinants usually causing lysis. Auto-immune haemolytic anaemia, idiopathic thrombocytopenic purpura are examples. Delayed hypersensitivity reaction - tuberculoid leprosy. Delayed hypersensitivity (type 4) reactions are mediated by primed T-lymphocytes; Tuberculoid leprosy, tuberculosis and contact dermatitis are examples. Asthma and infantile eczema are examples of atopy (type 1). 26. Which ONE of the following is not an organ specific auto-immune disease? A. Auto-immune adrenalitis. B. Chronic auto-immune gastritis. C. Chronic auto-immune thyroiditis. D. Insulin dependent diabetes. E. Rheumatoid arthritis. The answer is E. Rheumatoid arthritis is one of the group of connective tissue diseases with evidence for an auto-immune pathogenesis. Rheumatoid factor consists of IgM antibodies to altered IgG which is autoantigenic. In insulin dependent diabetes the trigger for the auto- immune reaction may be a viral infection. 27. For each of the types of immunological deficiency states listed on the left select the most appropriate association from those on the right. a. Di George syndrome. b. Infantile sex-linked agammaglobulinaemia. c. Severe combined immunodeficiency. 14 A. Defective B-cell function. B. Defective B- and T-cell function. C. Defective T-cell function. D. Defective platelets. E. Defective vessels. The answer is C, A, B. Di George syndrome - defective TT-cell function. There is almost complete failure of development of the thymus and parathyroids from the third and fourth branchial arches. Infantile sex-linked agammaglobulinaemia - defective B-cell function. This selective B-cell defect (Bruton type) results in failure to produce IgG, IgM and IgA. Severe combined immunodeficiency - defective B- and T-cell function. This combined type (Swiss type) of agammaglobulinaema has failure of development of both thymus dependent and thymus independent systems. Platelets are abnormal (D) in the rare Wiskott-Aldrich syndrome in which T-cell function, IgM and IgA are also abnormal. Abnormal vessels (E) are a feature of ataxia telangiectasia in which there is abnormal cell mediated immunity and low levels of IgA and IgE. 7. Infection 28. Which ONE of the following is not a feature of interferons? A. Imparts resistance to virus infection. B. Inhibits virtually all viruses. C. Released from cells in response to virus infection. D. Species specific cellular protein. E. Virus specific antiviral effect. The answer is E. The production of interferons is induced in host cells by virus; the interferons are not specific, but are an important host defence mechanism against virus infection. They act by inhibiting translation of viral mRNA by host cells. 29. For each of the features listed on the left select the most appropriate association from those on the right. a. Bacteraemia. b. Pyaemia. c. Septicaemia. A. Due to bacterial exotoxin. B. Fragment of septic thrombus. C. End result of viral infection. D. May be the result of vigorous teeth brushing. 15 E. Multiplication of bacteria in the blood. The answer is D, B, E. Bacteraemia - may be the result of vigorous teeth brushing. Bacteraemia is the presence of small numbers of bacteria in the blood; this can occur in normal individuals, i.e., after teeth brushing (NB this may be important in patients with valvular heart disease). Pyaemia - fragment of septic thrombus. Pyaemia (pus in blood) is the result of localized pyogenic infection damaging vascular endothelium and producing infected thrombus which breaks down. Septicaemia - multiplication of bacteria in the blood. Septicaemia is the presence and multiplication of organisms in the blood stream; this is the most serious type. Bacterial exotoxins (A) are produced by living bacteria. None of these is an end result of viral infection (C) since all are caused by bacteria. 30. Which one of the following is the best definition of gangrene? A. Digestion of dead tissue by saprophytic bacteria. B. Digestion of living tissue by saprophytic bacteria. C. Gas production in dead tissue. D. Necrosis of tissue caused by bacterial toxins. E. Necrosis of tissue caused by ischaemia. The answer is A. In gangrene, tissue which is dead is digested by bacteria which are incapable of invading and multiplying in living tissue (saprophytes). Gas production (C) may be present in some forms of gangrene particularly when caused by the anaerobic Clostridia. Necrosis of tissue is an essential prerequisite for gangrene, but it may be caused by ischaemia (E), i.e., secondary gangrene or by bacterial toxins (D), i.e., primary gangrene. 31. The pathogenicity of the tubercle bacillus is due to which ONE of the following? A. Ability to multiply within macrophages. B. Delayed hypersensitivity reaction against the bacteria. C. Direct toxic effect on host cells. D. Effective antibody response. E. Necrosis caused by expanding granulomas. The answer is B. Mycobacteria stimulate a specific T-cell response of cell mediated immunity; while this is effective in reducing the infection the delayed hypersensitivity reaction also damages the tissues. The tubercle bacilli have no demonstrable direct toxic action (C) and can survive within macrophages (A). This may account for latent infections and reactivation of tuberculosis. There is no significant humoral response to tubercle bacilli (D). Necrosis occurs in tuberculosis, but it is usually within the granuloma (E).