Internal Medicine_Infectious Diseases_1

Question 1

All Staphylococcus bacteria are _________ positive.

Answer 1

Catalase positive.

Question 2

What features does Staphylococcus aureus have that distinguish it from other bacteria?

Answer 2

Gram stain positive cocci in clusters

catalase positive (survives in oxygen)

coagulase positive
(converts fibrin to fibrinogen)

mannitol fermenter

beta hemolytic

Question 3

What differentiates Staphylococcus aureus from other Staphylococcus spp. ?

Answer 3

Staphylococcus aureus is coagulase positive.

This helps to distinguish Staphylococcus aureus from S. epidermidis and S. saprophyticus

Question 4

What is a primary virulence factor of Staphylococcus aureus that inhibits complement activation and phagocytosis?

Answer 4

Protein A, which binds the Fc region of IgG antibodies.

Question 5

Where is Staphylococcus aureus part of the normal flora?

Answer 5

The nares and skin (ears, axilla and groin).

Question 6

How does Staphylococcus aureus pneumonia typically present on a chest X-ray?

Answer 6

Patchy infiltrates that may progress to lobar consolidation.

Question 7

What condition can predispose individuals to Staphylococcus aureus pneumonia?

Answer 7

Upper respiratory infections, particularly post-influenza.

Question 8

What is the most common cause of septic arthritis?

Answer 8

Staphylococcus aureus.

Question 9

How does septic arthritis caused by Staphylococcus aureus present?

Answer 9

A warm, swollen, tender joint with purulent fluid on aspiration.

Usually more than 20 K WBC (> 75% PMNs)

Question 10

What types of skin and soft tissue infections are commonly caused by Staphylococcus aureus?

Answer 10

Abscesses
S aureus makes coagulase and toxins.
Forms fibrin clot around itself to produces an abscess.

Leads to: Impetigo, furuncles, carbuncles, and cellulitis.

Question 11

What heart condition caused by Staphylococcus aureus manifests with rapid-onset fever, chills, tachycardia, and sepsis?

Answer 11

Acute infective endocarditis.

Usually right-sided (tricuspid) heart infective endocarditis due to IV drug use.

Question 12

Which valve is most commonly affected in Staphylococcus aureus-associated infective endocarditis in IV drug users?

Answer 12

The tricuspid valve.

Question 13

What condition is caused by the exfoliative toxin produced by Staphylococcus aureus?

Answer 13

Scalded skin syndrome, characterized by widespread skin peeling due to the exfoliative toxin which is a protease.

Question 14

What toxin produced by Staphylococcus aureus causes toxic shock syndrome (TSS)?

Answer 14

Superantigen exotoxin which binds non specifically to MHC class II cells and TCs leading to a cytokine storm.

Question 15

What are the symptoms of toxic shock syndrome?

Answer 15

High fever, desquamative rash, and shock (low blood pressure and/or very high heart rate).

Question 16

What causes the rapid-onset form of food poisoning from Staphylococcus aureus?

Answer 16

Ingestion of preformed enterotoxin.

Question 17

What are the symptoms of Staphylococcus aureus food poisoning, and how quickly do they appear?

Answer 17

Sudden nausea, vomiting, and stomach cramps, typically developing within 30 minutes to 8 hours.

Question 18

What types of foods are commonly associated with Staphylococcus aureus enterotoxin contamination?

Answer 18

Mayonnaise and custards as well as meats.

Question 19

What has MRSA developed resistance to, and what is this strain called?

Answer 19

Resistance to beta-lactam antibiotics, including methicillin through the mecA gene.

Question 20

What is the mechanism of resistance in MRSA strains?

Answer 20

Modified penicillin-binding proteins (PBPs), which prevent beta-lactam antibiotics from binding to bacterial cell walls.

Question 21

What antibiotic is effective against MRSA infections?

Answer 21

Vancomycin.

Alternatively: Linezolid or daptomycin.

Question 22

What beta-lactam antibiotic is effective against non-MRSA Staphylococcus aureus infections?

Answer 22

Nafcillin or oxacillin is given to infections that are methicillin-sensitive.

Question 23

What test distinguishes Staphylococcus epidermidis and S. saprophyticus from Staphylococcus aureus?

Answer 23

They are coagulase-negative, whereas S. aureus is coagulase-positive.

When performing a coagulase test, the presence or absence of coagulase can be observed by adding a bacterial culture to a test tube containing coagulase-reacting plasma. If the bacteria are coagulase negative, a visible clot will not form in the plasma. Both Staphylococcus epidermidis and Staphylococcus saprophyticus are coagulase-negative, differentiating them from Staphylococcus aureus, which is coagulase-positive.

Question 24

What is the leading cause of osteomyelitis, and what does it involve?

Answer 24

Staphylococcus aureus, causing inflammation or swelling of the bone.

Staphylococcus aureus is the most common cause of osteomyelitis.

Other (less common causes) of osteomyelitis are:
- Streptococcus
- Coag Neg Staph (Puncture wounds and prothesis)
- Pseudomonas (Puncture wounds, IV drug use and immunocompromised)
- Fungal (IV drug use and immunocompromised)

Question 25

What are the risk factors associated with osteomyelitis?

Answer 25

IV drug use

Open fractures

Diabetes

Ulcers

Bacteremia

Question 26

A 10-year-old boy presents with a 2-week history of fever and worsening pain in his left tibia. On examination, he has tenderness and warmth over the tibia. Laboratory studies reveal leukocytosis and elevated ESR and CRP levels. X-ray of the tibia shows periosteal elevation. Blood cultures grow a Gram-positive cocci in clusters.

Which of the following is the most appropriate initial management?

A) Oral clindamycin
B) Intravenous vancomycin
C) Intravenous ceftriaxone
D) Surgical debridement
E) Observation only

Answer 26

Answer: B) Intravenous vancomycin

Explanation: Hematogenous osteomyelitis in children is most commonly caused by Staphylococcus aureus. Additionally, hematogenous spread is the most common cause of osteomyelitis in children. Empiric treatment should include antibiotics effective against methicillin-resistant Staphylococcus aureus (MRSA), such as vancomycin. If methicillin-sensitive Staphylococcus aureus (MSSA) is confirmed, therapy can be de-escalated to cefazolin.

Question 27

A 12-year-old boy with a history of sickle cell disease presents with fever and right femur pain for 5 days. Physical examination reveals swelling and tenderness over the distal femur. Laboratory studies show an elevated WBC count and ESR.

Which of the following organisms is the most likely cause of his osteomyelitis?

A) Staphylococcus aureus
B) Salmonella species
C) Escherichia coli
D) Pseudomonas aeruginosa
E) Mycobacterium tuberculosis

Answer 27

Answer: B) Salmonella species

Explanation: In patients with sickle cell disease, Salmonella species is the most common cause of osteomyelitis, although Staphylococcus aureus is also common. Salmonella has a propensity to invade areas of bone infarction, which is frequent in sickle cell patients due to vaso-occlusive crises.

Question 28

A 60-year-old man with a history of diabetes mellitus presents with a chronic non-healing ulcer over the plantar surface of his foot. He reports increased swelling, redness, and discharge from the ulcer over the last 2 weeks. A probe-to-bone test is positive. MRI confirms osteomyelitis in the metatarsals.

What is the next best step in management?

A) Start empiric antibiotics and await blood culture results
B) Perform a bone biopsy for culture and histopathology
C) Amputate the affected foot
D) Obtain an X-ray of the foot
E) Prescribe oral antibiotics targeting Gram-negative organisms

Answer 28

Answer: B) Perform a bone biopsy for culture and histopathology

Explanation: In diabetic foot ulcers with osteomyelitis, identifying the causative organism is crucial for targeted antibiotic therapy. Bone biopsy provides a definitive diagnosis and guides antibiotic selection. Empiric antibiotics can be started after the biopsy if clinically indicated.

Question 29

A 35-year-old man who uses intravenous drugs presents with severe back pain for 3 weeks. He has no history of trauma. On physical examination, he has midline tenderness over the lower thoracic spine. ESR is 90 mm/hr, and blood cultures are positive for a Gram-positive cocci in clusters. MRI shows destruction of the T8 and T9 vertebral bodies with a paravertebral abscess.

Which of the following is the most likely diagnosis?

A) Osteomyelitis caused by Pseudomonas aeruginosa
B) Vertebral osteomyelitis caused by Staphylococcus aureus
C) Osteomyelitis caused by Salmonella species
D) Tuberculous osteomyelitis (Pott’s disease)
E) Osteomyelitis caused by Escherichia coli

Answer 29

Answer: B) Vertebral osteomyelitis caused by Staphylococcus aureus

Explanation: Staphylococcus aureus is the most common cause of vertebral osteomyelitis, particularly in IV drug users. Blood cultures and imaging are key for diagnosis, and MRI is the most sensitive imaging modality for detecting vertebral involvement and abscesses.

Question 30

A 40-year-old woman presents 3 weeks after undergoing open reduction and internal fixation of a tibial fracture. She now has pain, swelling, and redness at the surgical site. The surgical wound has purulent drainage, and imaging reveals periosteal reaction consistent with osteomyelitis.

What is the most important next step in management?

A) Empiric antibiotics with Gram-positive and Gram-negative coverage
B) Removal of hardware and surgical debridement
C) Perform blood cultures and start oral antibiotics
D) Immobilize the limb and observe
E) Amputation of the affected limb

Answer 30

Answer: B) Removal of hardware and surgical debridement

Explanation: Post-surgical osteomyelitis often requires both surgical debridement to remove infected tissue and removal of the hardware if it is the source of infection. Antibiotics alone are insufficient without addressing the infected hardware.

Question 31

What type of bacteria are Staphylococcus epidermidis and Staphylococcus saprophyticus?

Answer 31

Both are gram-positive cocci.

Question 32

What test distinguishes Staphylococcus epidermidis and S. saprophyticus from Staphylococcus aureus?

Answer 32

They are coagulase-negative, whereas S. aureus is coagulase-positive.

When performing a coagulase test, the presence or absence of coagulase can be observed by adding a bacterial culture to a test tube containing coagulase-reacting plasma. If the bacteria are coagulase negative, a visible clot will not form in the plasma. Both Staphylococcus epidermidis and Staphylococcus saprophyticus are coagulase-negative, differentiating them from Staphylococcus aureus, which is coagulase-positive.

Question 33

What enzyme do S. epidermidis and S. saprophyticus produce to convert hydrogen peroxide into water and oxygen that helps distinguish staphylococcus from streptococcus ?

Answer 33

Catalase.

Staphylococcus epidermidis and Staphylococcus saprophyticus are catalase-positive. The production of this enzyme helps differentiate staphylococci from streptococci, which are catalase-negative.

Question 34

What enzyme produced by S. epidermidis and S. saprophyticus hydrolyzes urea into carbon dioxide and ammonia?

Answer 34

Urease.

Question 35

Where is S. epidermidis commonly found in the body?

Answer 35

It is part of the normal flora of the skin and mucous membranes.

Question 36

Why is S. epidermidis a common contaminant of blood cultures?

Answer 36

Because it is part of the skin flora.

Question 37

What medical devices is S. epidermidis associated with due to biofilm production?

Answer 37

• Prosthetic joints (causing prosthetic joint infections).
• Catheters (causing catheter-associated infections).
• Prosthetic heart valves (causing prosthetic valve endocarditis).

Question 38

What substance allows S. epidermidis to adhere to prosthetic devices and evade the immune system?

Answer 38

Biofilms (extracellular polysaccharide matrix).

Question 39

How can S. epidermidis be distinguished from S. saprophyticus in the lab?

Answer 39

S. epidermidis is novobiocin-sensitive, whereas S. saprophyticus is resistant.

Question 40

What is the drug of choice for treating S. epidermidis infections?

Answer 40

Vancomycin (due to frequent resistance to nafcillin and methicillin), especially for endocarditis.

Question 41

What infections are commonly caused by S. saprophyticus?

Answer 41

• Uncomplicated urinary tract infections in young, sexually active females (honeymoon cystitis).
• Acute bacterial prostatitis.

Question 42

What is S. saprophyticus resistant to that helps distinguish it from S. epidermidis?

Answer 42

Novobiocin.

Novobiocin susceptibility can be used to differentiate between the two.
Staphylococcus epidermidis is sensitive to novobiocin, whereas Staphylococcus saprophyticus is resistant.

Question 43

What pathogens are most commonly implicated in Central line-associated bloodstream infection?

Answer 43

Staphylococci such as S. aureus account for the majority of cases of healthcare-associated IE.

Rapid progression and an acute onset of symptoms is usually S. aureus. Acute IE due to S. aureus can quickly lead to complications such as valvular insufficiency, heart failure, and septic emboli. Therefore, in addition to removing the infected CVC, empiric antibiotic therapy (vancomycin plus beta-lactam for native valve endocarditis) should be initiated after obtaining blood cultures.

Coagulase-negative staphylococci (e.g., S. epidermidis), S. aureus, enterococci, and Candida spp. are the organisms most commonly associated with CLABSI with a more insidious progression (weeks to months) with low-grade fever and nonspecific symptoms.