MCQs – Infections 1

Modern Study Review (AI-Generated)

High-Yield Summary

This topic is a staple of the Royal College exam, focusing heavily on infectious orthopaedics, prophylaxis protocols, and pathogen-specific clinical features. The single most important clinical trade-off is the immunization status and wound type dictating tetanus prophylaxis strategy, which often determines the correct management pathway. While classic exam answers emphasize rigid prophylaxis algorithms, modern practice increasingly integrates risk stratification and local epidemiology, especially regarding antibiotic stewardship and infection control. Understanding pathogen-specific virulence factors and resistance patterns is critical for targeted therapy, particularly in prosthetic joint infections and bite wounds.

High-Yield Decision Matrix

Active Recall Q&A

Herpes and Viral Infections

Q: Which viruses belong to the herpes family relevant to orthopaedic infections?
A: HSV-1, HSV-2, varicella-zoster, cytomegalovirus, Epstein-Barr virus.
Related Pearl: HSV-1 commonly causes herpetic whitlow, an occupational hazard for healthcare workers, often misdiagnosed as bacterial infection.

Q: What clinical features characterize HSV-1 infections?
A: Gingival stomatitis, cold sores, corneal keratitis, and painful pustules on erythematous bases (herpetic whitlow).
Related Pearl: Herpetic whitlow mimics bacterial infections but requires antiviral treatment; early recognition prevents unnecessary antibiotics.

Bacterial Infections

Q: What is the most common cause of flexor tenosynovitis?
A: Staphylococcus aureus.
Related Pearl: Prompt diagnosis and treatment are essential to prevent tendon necrosis and preserve hand function.

Q: What is the risk of HIV transmission after a needle-stick injury?
A: Approximately 0.3% to 0.5%.
Related Pearl: Immediate post-exposure prophylaxis significantly reduces seroconversion risk.

Q: What distinguishes streptococcal necrosis from clostridial necrosis?
A: Clinical differentiation is important due to differing treatment and prognosis, though specific features were not detailed.
Related Pearl: Clostridial infections produce gas and rapid tissue destruction, requiring urgent surgical debridement.

Q: What is the most common complication of neonatal osteomyelitis?
A: Growth arrest due to multiple physeal involvement.
Related Pearl: Early intervention is critical to prevent limb length discrepancies and deformities.

Q: Which antibiotics are associated with C. difficile colitis, and what is the typical onset?
A: Ampicillin, clindamycin, and cephalosporins; symptoms may present up to two weeks after treatment.
Related Pearl: Oral vancomycin or metronidazole are first-line treatments for C. difficile colitis.

Q: What is the mechanism of action of ciprofloxacin, and why is it contraindicated in children?
A: Inhibits DNA gyrase, essential for DNA replication; contraindicated in children due to cartilage damage risk demonstrated in animal studies.
Related Pearl: Ciprofloxacin absorption is reduced by Mg/Al antacids and interacts with theophylline and warfarin, requiring careful medication review.

Q: What toxin does Clostridium perfringens produce, and what is its clinical effect?
A: Alpha toxin, an exotoxin that lyses red blood cell membranes causing hemolytic anemia.
Related Pearl: This toxin contributes to the rapid systemic toxicity and hemolysis seen in gas gangrene.

Tetanus Prophylaxis

Q: What is the tetanus prophylaxis regimen for an unimmunized patient?
A: Full Td vaccination series.
Related Pearl: Unimmunized patients with tetanus-prone wounds require both active and passive immunization.

Q: What is the tetanus prophylaxis for a patient not fully immunized with a tetanus-prone wound?
A: Both tetanus immune globulin (TIG) and Td vaccine.
Related Pearl: TIG provides immediate passive immunity; Td induces long-term active immunity.

Q: What is the tetanus prophylaxis for a fully immunized patient with a tetanus-prone wound within the last 5 years?
A: No additional treatment needed.
Related Pearl: Immunity is considered protective within 5 years of the last booster.

Q: What is the tetanus prophylaxis for a fully immunized patient with a tetanus-prone wound more than 5 years after last booster?
A: Td booster vaccination.
Related Pearl: Immunity wanes over time; boosters are recommended every 10 years or sooner with tetanus-prone wounds.

Q: What is the tetanus prophylaxis for a fully immunized patient with a clean wound within 10 years?
A: No treatment needed.
Related Pearl: Clean wounds have minimal risk for tetanus in fully immunized patients.

Q: When should both Td and TIG be administered if tetanus status is unknown?
A: Administer 0.5 mL Td and 250 units TIG; this is the only indication for TIG alone.
Related Pearl: TIG provides immediate passive immunity when immunization history is uncertain.

Periprosthetic and Bite Infections

Q: What role does fibronectin play in periprosthetic infections?
A: Fibronectin in the endothelial glycocalyx binds gram-positive bacteria like S. aureus and streptococci, facilitating infection.
Related Pearl: Surgical trauma exposes fibronectin, increasing bacterial adhesion and biofilm formation risk.

Q: Which bacteria are commonly associated with human and dog bite infections?
A: Eikenella corrodens (gram-negative rod).
Related Pearl: Eikenella is penicillin-sensitive, unlike many other bite wound pathogens.

Q: Which bacteria is most common in cat bite infections?
A: Pasteurella species.
Related Pearl: Pasteurella infections can progress rapidly and require prompt antibiotic therapy.

Q: What is the most common organism in human bite infections?
A: Streptococcus viridans.
Related Pearl: Human bites often cause polymicrobial infections, necessitating broad-spectrum antibiotics.

Septic Arthritis and Hospital Infections

Q: What is the most common joint affected by septic arthritis?
A: The knee.
Related Pearl: Joint aspiration is critical for diagnosis and guides targeted therapy.

Q: How do bacteria protect themselves from host defenses and antibiotics?
A: By producing an extracellular glycocalyx matrix.
Related Pearl: This biofilm complicates eradication, especially in prosthetic joint infections.

Q: What is the significance of Serratia infections in hospitals?
A: Serratia is an opportunistic pathogen spread by hospital staff hands and may cause arthroscopy infections.
Related Pearl: Strict hand hygiene is essential to prevent nosocomial infections.

Hip Arthroplasty Infections

Q: What is the treatment for Staphylococcus epidermidis infection of the hip?
A: Vancomycin.
Related Pearl: S. epidermidis is often methicillin-resistant and a common cause of late prosthetic joint infections.

Q: What is the most common organism in recurrent hip infections?
A: Staphylococcus aureus.
Related Pearl: S. aureus is highly virulent, causing early and recurrent infections.

Q: Which organism is the most difficult to eradicate in hip arthroplasty infections?
A: Pseudomonas species.
Related Pearl: Pseudomonas forms biofilms and exhibits multidrug resistance, complicating treatment.

Q: What is the best diagnostic method for hip infection?
A: Aspiration of the hip joint.
Related Pearl: Synovial fluid analysis and culture guide targeted antibiotic therapy.

Foot Puncture Wounds and Other Infections

Q: Which organisms should be suspected in puncture wounds to the foot?
A: Staphylococcus aureus and Pseudomonas species.
Related Pearl: Pseudomonas is especially likely if the nail penetrates the shoe.

Q: What is characteristic of pseudomonas infection discharge?
A: Colorless discharge rather than purulent pus.
Related Pearl: This subtle presentation can delay diagnosis and treatment.

Q: Which patient populations are typically affected by Listeria monocytogenes infections?
A: Neonates and immunocompromised patients.
Related Pearl: Listeria is intracellular and requires specific antibiotic coverage.

Q: How is Flagyl (metronidazole) metabolized and excreted?
A: Metabolized by the liver and excreted by the kidneys; kidney dysfunction does not prolong half-life.
Related Pearl: Liver impairment affects dosing more than renal impairment.

Classic Clinical Notes

• Herpes simplex infections are HSV-1 and HSV-2, although varicella-zoster, cytomegalovirus, Epstein-Barr are all part of the herpes family.

• HSV-1 is typically “above the waist” – gingival stomatitis, cold sores, corneal keratitis, and skin infections of the finger in which it appears as a painful pustule on an erythematous base – “herpetic whitlow”. This is an occupational hazard of nurses, dentists, physicians, and lab technicians, usually resulting from inoculation of infected secretions through a small cut in the skin. It is frequently misdiagnosed as staph or strep infection.

• Most common cause of flexor tenosynovitis is staph aureus.

• HIV transmission post needle-stick is around 0.3 to 0.5%.

• It seems important to be able to distinguish between strep necrosis and clostridial necrosis.

• The most common complication of neonatal osteo is growth arrest from multiple physeal involvement.

• C. difficile can be treated with flagyl or oral vanco. Colitis can be caused by amp, clinda, and cephalosporins, and may present up to two weeks after treatment.

• Ciprofloxacin inhibits DNA gyrase, an essential component of the DNA replication system; it does not work on the cell wall. Cipro has been shown in animals to cause damage to cartilage and therefore should not be used in growing children with growth plates still open. It is effective against many gram negative organisms. Its absorption is inhibited by magnesium or aluminum containing antacids, and it interacts with theophyline and coumadin.

• C. perfringens produces alpha toxin (an exotoxin) which breaks down cell membranes, including red blood cell membranes – causing hemolysis. Therefore, C. perfringens is most likely to cause a hemolytic anemia.

• Know the indications for tetanus prophylaxis:
• Basically, if unimmunized, they need the Td regimen.
• If not immunized fully and has a tetanus prone wound – get TIG and Td.
• If immunized fully within the past 5 years with a tetanus prone wound – nothing.
• If immunized fully but more than 5 years ago, with a tetanus prone wound – Td.
• If immunized fully within the past 10 years with a clean wound – nothing.
• If unsure of tetanus status – give both 0.5 mL Td and 250 units TIG. This is the only circumstance to use TIG.

• Fibronectin has a role in periprosthetic infections – fibronectin is part of the endothelial cell glycocalyx, and may be exposed when there is local damage (from putting in the joint?). Some gram-positive bacteria, including S. aureus, St. pyogenes, St mutans, and St sanguis, bind to fibronectin.

• Flagyl is metabolized by the liver and excreted by the kidney. The metabolism is in the liver – altered kidney function does not prolong the half life.

• Eikenella corrodons infections are common with human and dog bites (pasteurella is with cat bites). Eikenella is a gram negative rod and is best treated with penicillin.

• The most common organism in human bites is strep viridans.

• The most common joint in septic arthritis is probably the knee.

• Bacteria can exude an extracellular glycocalyx matrix which protects them from host defenses and antibiotics.

• Serratia infection seems to be an opportunistic infection that is most commonly spread by hands of hospital employees and may be a cause of infection in arthroscopy.

• Staph epi of the hip is treated with Vancomycin.

• The most common organism in recurrent hip infection is staph aureus.

• The most difficult bug to kill in hip arthroplasty is pseudomonas.

• Aspiration of the hip is the best way to diagnose infection.

• The most common cause of latent infection of the total hip (6 months out) is staph epi.

• In puncture wounds to the foot, beware of staph and pseudomonas infections. If the nail goes through the shoe, be particularly wary of pseudomonas, which shows up with a colourless discharge rather than pus.

• Listeria monocytogenes infection is typically an infection of neonates and immunocompromised.

Last Updated on January 25, 2026 by Christian Veillette