Untitled Study Note

Question 1

Which one of the following is a complication of streptococcus pharyngitis?

Accepted Answer

Rheumatic fever — Rheumatic fever is a nonsuppurative complication of Group A Streptococcus (Streptococcus pyogenes) pharyngitis. It occurs 2-4 weeks after untreated strep throat due to molecular mimicry, where antibodies against streptococcal M protein cross-react with cardiac tissue, causing carditis, arthritis, and chorea. ---

Question 2

The presence of a pseudomembrane is a result of

Accepted Answer

Diphtheria — Corynebacterium diphtheriae produces diphtheria toxin that causes local tissue necrosis in the pharynx, forming a characteristic gray-white pseudomembrane composed of fibrin, dead cells, bacteria, and inflammatory exudate. This membrane adheres firmly to underlying tissue and can cause airway obstruction. ---

Question 3

A paroxysm is a and is characteristic of

Accepted Answer

A paroxysm — A paroxysm is a sudden, uncontrollable series of coughs characteristic of pertussis (whooping cough) caused by Bordetella pertussis. The paroxysmal stage features severe coughing fits followed by an inspiratory "whoop" sound, often accompanied by post-tussive vomiting. ---

Question 4

The Mantoux test is used to identify

Accepted Answer

TB exposure — The Mantoux test (tuberculin skin test) detects delayed-type hypersensitivity to tuberculin purified protein derivative (PPD), indicating prior exposure to Mycobacterium tuberculosis. A positive result shows induration at the injection site 48-72 hours after intradermal injection. ---

Question 5

This bacterial species is the most common cause of "typical" pneumonia.

Accepted Answer

S. pneumoniae — Streptococcus pneumoniae (pneumococcus) is the most common cause of community-acquired bacterial pneumonia. It presents with typical features: acute onset, high fever, productive cough with rusty sputum, pleuritic chest pain, and lobar consolidation on chest X-ray. ---

Question 6

To treat patients who have ingested botulism toxin, large doses of must be administered.

Accepted Answer

Antibiotic — Botulism requires administration of botulinum antitoxin (trivalent or heptavalent). The antitoxin neutralizes circulating toxin but cannot reverse already bound toxin. Early administration is critical. Antibiotics are ineffective against the preformed toxin. ---

Question 7

The organism Clostridium perfringens multiplies in foods only under conditions.

Accepted Answer

Aerobic — Clostridium perfringens is an obligate anaerobe that multiplies only in oxygen-free environments. Improper food storage (inadequate cooling or reheating) creates anaerobic conditions that allow spore germination and toxin production, leading to food poisoning. ---

Question 8

A spore-forming, motile rod that can cause foodborne illness in a diarrheal or emetic form is Bacillus

Accepted Answer

Cereus — Bacillus cereus causes two forms of food poisoning: the emetic form (1-6 hours, associated with fried rice) caused by cereulide toxin, and the diarrheal form (8-16 hours, associated with meats and vegetables) caused by enterotoxins. ---

Question 9

Woolsorter disease applies to the form of

Accepted Answer

Inhalation; anthrax — Woolsorter's disease is pulmonary anthrax caused by inhaling Bacillus anthracis spores from contaminated animal products (wool, hides). It causes severe hemorrhagic mediastinitis, septic shock, and high mortality. The name comes from textile workers who processed contaminated wool. ---

Question 10

Which one of the following describes the mode of action of tetanospasmin?

Accepted Answer

It inhibits muscle relaxation — Tetanospasmin (tetanus toxin) blocks release of inhibitory neurotransmitters (GABA and glycine) from Renshaw cells in the spinal cord by cleaving synaptobrevin. This prevents muscle relaxation, causing sustained muscle contraction (spastic paralysis) and characteristic lockjaw. ---

Question 11

Salpingitis is associated with and can lead to

Accepted Answer

Gonorrhea; sterility — Salpingitis (fallopian tube inflammation) is commonly caused by Neisseria gonorrhoeae as part of pelvic inflammatory disease (PID). Chronic inflammation causes scarring and tubal obstruction, leading to infertility and increased ectopic pregnancy risk. ---

Question 12

Ophthalmia is associated with what two STDs?

Accepted Answer

Gonorrhea and chlamydia — Ophthalmia neonatorum (neonatal conjunctivitis) is caused by Neisseria gonorrhoeae and Chlamydia trachomatis acquired during passage through an infected birth canal. Both can cause severe conjunctivitis and, if untreated, corneal scarring and blindness. ---

Question 13

Toxin-generated S. aureus diseases resulting from contact include

Accepted Answer

Scalded skin syndrome — Staphylococcus aureus produces exfoliative toxins causing scalded skin syndrome (bullous impetigo, Ritter's disease) and toxic shock syndrome toxin-1 (TSST-1) causing toxic shock syndrome. Both are toxin-mediated diseases that can result from skin colonization or infection. ---

Question 14

The first microorganism to satisfy Koch's postulates (in the late 19th century) was

Accepted Answer

Bacillus anthracis — Robert Koch first proved Bacillus anthracis as the causative agent of anthrax in 1876, establishing Koch's postulates. He isolated the organism from diseased animals, cultured it in pure form, reproduced the disease in healthy animals, and re-isolated the same organism. ---

Question 15

Which of the following statements about lipopolysaccharide is correct?

Accepted Answer

It interacts with macrophages and monocytes yielding release of cytokines — Lipopolysaccharide (LPS/endotoxin) binds to TLR4 on macrophages and monocytes via LPS-binding protein and CD14, triggering massive cytokine release (TNF-α, IL-1, IL-6). This causes fever, shock, and DIC. The toxic component is Lipid A, not the O antigen. ---

Question 16

Which of the following statements is correct?

Accepted Answer

coli is a gram-negative bacterium with LPS in its outer membran — E. coli is a gram-negative bacterium with LPS in its outer membrane. Cholera toxin is secreted (not on flagella), lecithinase causes tissue necrosis (not diarrhea), and TSST-1 is produced by S. aureus (not S. epidermidis). ---

Question 17

Which of the following best describes the mechanism of action of diphtheria toxin?

Accepted Answer

Diphtheria toxin — Diphtheria toxin is an A-B toxin. The A subunit catalyzes ADP-ribosylation of elongation factor 2 (EF-2), irreversibly inactivating it and blocking protein synthesis. One toxin molecule can kill a cell by halting translation. ---

Question 18

Certain microorganisms are never considered to be members of the normal flora. They are always considered to be pathogens. Which one of the following organisms fits into that category?

Accepted Answer

M. tuberculosis — M. tuberculosis is always pathogenic and never part of normal flora. The others can be normal flora: S. pneumoniae (nasopharynx), E. coli (intestine), S. aureus (skin/nose), N. meningitidis (nasopharynx up to 10% of population). ---

Question 19

An outbreak of sepsis caused by Staphylococcus aureus has occurred in the newborn nursery. You are called upon to investigate. According to your knowledge of the normal flora, what is the most likely source of the organism?

Accepted Answer

Nose — The anterior nares (nose) is the primary reservoir for S. aureus colonization, occurring in 20-40% of healthy adults. Healthcare workers can transmit the organism from their nose to patients via hands, making nasal carriage the most likely source in nosocomial outbreaks. ---

Question 20

Each of the following agent is a recognized cause of diarrhea except?

Accepted Answer

Enterococcus faecalis — Enterococcus faecalis is normal intestinal flora and causes urinary tract infections, endocarditis, and intra-abdominal infections, but not diarrhea. C. perfringens, V. cholerae, and E. coli are all well-recognized diarrheal pathogens. ---

Question 21

The identification of bacteria by serological test is based on the presence of specific antigens. Which of the following bacterial components is least likely to contain useful antigens?

Accepted Answer

Capsule — Ribosomes are internal structures with highly conserved proteins across species, making them poor serological markers. Capsule (K antigens), cell wall (O antigens for LPS), and flagella (H antigens) are surface-exposed and variable, making them excellent for serotyping. ---

Question 22

Scalded skin syndrome is caused by exotoxin (exfoliatins) produced by

Accepted Answer

Staphylococcus aureus — Staphylococcal scalded skin syndrome (SSSS) is caused by exfoliative toxins (ETA and ETB) produced by certain S. aureus strains. These serine proteases cleave desmoglein-1 in the epidermis, causing superficial skin separation and blistering resembling scalding. ---

Question 23

Which of the following bacteria has the lowest 50% infective dose (ID50)?

Accepted Answer

Salmonella requires ~10^5 organisms, V. cholerae ~10^8, and — Shigella has the lowest ID50 (10-100 organisms) due to acid resistance, allowing survival through stomach acid. Salmonella requires ~10^5 organisms, V. cholerae ~10^8, and C. jejuni ~500-10,000 organisms for infection. ---

Question 24

Which of the following disease is best diagnosed by serologic means?

Accepted Answer

Gonorrhea — Q fever (Coxiella burnetii) is best diagnosed serologically by detecting antibodies to phase I and II antigens. The organism is difficult to culture due to obligate intracellular nature. TB, gonorrhea, and actinomycosis are diagnosed by culture/microscopy. ---

Question 25

A 30 year old woman has non-bloody diarrhea for the past 14 hours. Which one of the following organisms is least likely to cause this illness?

Accepted Answer

difficile, Shigella, and Salmonella are all common causes of diarrhe — S. pyogenes causes pharyngitis, skin infections, and toxic shock syndrome, but not gastroenteritis. C. difficile, Shigella, and Salmonella are all common causes of diarrhea. Note: Shigella typically causes bloody diarrhea but can present with watery diarrhea initially. ---

Question 26

A 20 year old woman who reports unprotected sex with a new partner 2 weeks previously develop fever and left lower quadrant abdominal pain with onset in association with her menstrual period. The diagnosis is gonococcal pelvic inflammatory disease. What is the common sequela of this infection?

Accepted Answer

Infertility — Pelvic inflammatory disease (PID) from N. gonorrhoeae causes tubal scarring and adhesions, leading to infertility (10-20% after one episode, higher with recurrent infections) and increased ectopic pregnancy risk. Chronic pelvic pain is also common. ---

Question 27

The survival of Mycobacterium after ingestion by macrophages is attributed to

Accepted Answer

Bacterial inhibition of phagosome formation and interference with endosomal acidification — Mycobacterium tuberculosis prevents phagosome-lysosome fusion and blocks phagosome acidification through various mechanisms including production of ammonia and inhibition of vacuolar ATPase. This allows intracellular survival within macrophages. ---

Question 28

The molecular basis for effect of cholera toxin on duodenal mucosa cells is

Accepted Answer

Ribosylation of guanosine triphosphate (GTP) binding protein — Cholera toxin's A subunit ADP-ribosylates the Gsα protein (GTP-binding protein), locking it in the active state. This permanently activates adenylate cyclase, causing continuous cAMP production, which drives massive chloride and water secretion into the intestinal lumen. ---

Question 29

The role of bacteria capsule as virulence factor is usually related to their ability to interfere with

Accepted Answer

Phagocytosis — Bacterial capsules (e.g., S. pneumoniae, H. influenzae, N. meningitidis) are antiphagocytic. The hydrophilic polysaccharide layer prevents complement C3b deposition and blocks phagocyte receptors from binding bacterial surface, inhibiting opsonization and phagocytosis until specific anticapsular antibodies develop. ---

Question 30

Which of the following is not most important cause of acute bacterial meningitis?

Accepted Answer

Streptococcus pyrogens — S. pyogenes (Group A Strep) rarely causes meningitis. The three most common causes of bacterial meningitis are S. pneumoniae (most common overall), N. meningitidis (especially in young adults), and H. influenzae type b (reduced since Hib vaccine). ---

Question 31

The coagulase test is used to differentiate

Accepted Answer

from coagulase-negative staphylococci like S. epidermidis. Coagulase converts fibrinogen to fibrin, causing — The coagulase test distinguishes S. aureus (coagulase-positive) from coagulase-negative staphylococci like S. epidermidis. Coagulase converts fibrinogen to fibrin, causing plasma clotting. It's a key virulence factor and diagnostic marker for S. aureus. ---

Question 32

The organism most likely to be responsible for the patient's disease (Question 39) is

Accepted Answer

S. aureus producing TSST-1 — S. aureus producing TSST-1 causes toxic shock syndrome. Nasal packing creates ideal conditions for bacterial growth and toxin production. The clinical picture (fever, rash, hypotension, multi-organ involvement) is classic for staphylococcal TSS. ---

Question 33

Which of the following is most likely to be associated with the formation of a bacterial biofilm?

Accepted Answer

Mucoid P. aeruginosa — Mucoid P. aeruginosa produces alginate exopolysaccharide that forms thick biofilms in CF patient airways. These biofilms protect bacteria from antibiotics and immune clearance, causing chronic infection. Biofilm formation is a hallmark of CF pulmonary infection. ---

Question 34

A 15-year-old girl develops severe watery diarrhea. The stool looks like "rice water." It is voluminous—more than 1 L in the last 90 minutes. She has no fever and seems otherwise normal except for the effects of loss of fluid and electrolytes. The most likely cause of her illness is

Accepted Answer

This — This is cholera caused by V. cholerae producing cholera toxin ( an A-B toxin). The "rice-water" appearance, massive volume ( 1L/hour possible), absence of fever, and secretory nature are pathognomonic for cholera. The A subunit ADP-ribosylates Gs protein, causing massive fluid secretion. ---

Question 35

The most important thing that can be done to treat the patient (Question 43) is

Accepted Answer

To treat her with fluid and electrolyte replacement — Immediate oral rehydration therapy (ORT) or IV fluids is life-saving in cholera. Fluid replacement addresses the primary pathology (dehydration and electrolyte loss). Antibiotics (doxycycline or azithromycin) shorten duration but are secondary. Most deaths from cholera are due to inadequate rehydration. ---

Question 36

A 23-year-old woman has a history of recurrent urinary tract infections, including at least one episode of pyelonephritis. Blood typing shows the P blood group antigen. Which of the following is likely to be the primary cause of her infections?

Accepted Answer

Escherichia coli with P-pili (fimbria — P pili (P fimbriae) on uropathogenic E. coli specifically bind to P blood group antigens on uroepithelial cells. Patients with P antigens are more susceptible to pyelonephritis. P-piliated E. coli strains cause 90% of pyelonephritis cases in otherwise healthy individuals. ---

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