Skin, soft tissue and bone

11 ASCP MLS practice questions on Skin, soft tissue and bone, part of Microbiology. The first 11 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 20-month-old boy has a limp and a warm, swollen knee 1 week after a cold. Synovial fluid has 48,000 WBC/µL [48 x 10^9/L], 90% neutrophils, and the Gram stain shows no organisms. Plates inoculated directly from the fluid have no growth at 48 hours, but an aerobic blood culture bottle inoculated with the same fluid flags positive on day 3. The broth shows plump gram-negative coccobacilli in pairs and short chains. Subcultures grow small colonies with a narrow zone of beta-hemolysis, some of which pit the blood agar; there is no growth on MacConkey agar. The isolate is oxidase positive and catalase negative. The most likely organism is:
1. AMoraxella catarrhalis
2. BKingella kingae
3. CHaemophilus influenzae
4. DEikenella corrodens
Show answerHide answer
Answer: B · Kingella kingae
Kingella kingae is the leading cause of septic arthritis and osteomyelitis in children aged 6 months to 4 years, often after a viral upper respiratory infection. It is fastidious and often missed on directly inoculated plates, so inoculating joint fluid into a blood culture bottle, or using PCR, greatly improves detection. It is an oxidase-positive, catalase-negative coccobacillus that gives a narrow zone of beta-hemolysis and may pit the agar. Eikenella corrodens also pits agar and is catalase negative, but it is nonhemolytic, smells of bleach and follows human bites. H. influenzae and M. catarrhalis are catalase positive, and H. influenzae does not grow on sheep blood agar.
- A: M. catarrhalis is a catalase-positive, nonhemolytic diplococcus.
- C: H. influenzae is catalase positive and does not grow on sheep blood agar.
- D: Eikenella is nonhemolytic, smells of bleach and follows bite wounds.
Question 2 · Hard
A fish handler has a painful violet swelling on a finger. A biopsy grows a catalase-negative, nonmotile gram-positive bacillus that produces H2S in the TSI butt and is resistant to vancomycin. What is the most likely organism?
1. AListeria monocytogenes
2. BLactobacillus species
3. CErysipelothrix rhusiopathiae
4. DArcanobacterium haemolyticum
Show answerHide answer
Answer: C · Erysipelothrix rhusiopathiae
Erysipelothrix rhusiopathiae causes erysipeloid, a painful violaceous skin lesion, usually on the hands of people who handle fish, shellfish, poultry or meat. It is the only catalase-negative gram-positive rod of clinical importance that blackens the TSI butt, and it may show a 'test-tube brush' pattern in a gelatin stab. It is intrinsically resistant to vancomycin, which matters if it causes bacteremia or endocarditis. Penicillin is the treatment of choice.
- A: Listeria is catalase positive, motile and H2S negative.
- B: Lactobacillus is vancomycin resistant but does not produce H2S.
- D: Arcanobacterium is beta-hemolytic and H2S negative.
Question 3 · Easy
Which organism is most often isolated from an infected cat bite?
1. ACapnocytophaga canimorsus
2. BBartonella henselae
3. CPasteurella multocida
4. DEikenella corrodens
Show answerHide answer
Answer: C · Pasteurella multocida
Pasteurella multocida lives in the mouths of most cats and many dogs, and cat bites, which are deep puncture wounds, often become infected within 24 hours. It is a small gram-negative coccobacillus that is oxidase, catalase and indole positive, grows on blood and chocolate agar but not on MacConkey, and has a musty odor. It is susceptible to penicillin, which is unusual for a gram-negative rod. Bite wounds are often polymicrobial, with anaerobes and staphylococci as well.
- A: Capnocytophaga canimorsus is mainly linked to dog bites and sepsis in asplenic patients.
- B: Bartonella henselae causes cat-scratch disease, with lymph node swelling weeks later.
- D: Eikenella is linked to human bites and clenched-fist injuries.
Question 4 · Easy
Yellow 'sulfur granules' in pus from a draining abscess of the jaw suggest infection with:
1. AActinomyces israelii
2. BFusobacterium nucleatum
3. CCutibacterium acnes
4. DPrevotella melaninogenica
Show answerHide answer
Answer: A · Actinomyces israelii
Sulfur granules are clumps of branching gram-positive bacilli held together in a matrix, typical of actinomycosis. Actinomyces israelii is part of the normal oral flora and causes slowly spreading cervicofacial abscesses with sinus tracts, often after dental work or jaw injury. Crushed granules show branching gram-positive rods that are negative with the modified acid-fast stain, unlike Nocardia. The organism is anaerobic to microaerophilic and grows slowly, forming 'molar tooth' colonies after a week or more.
- B: Fusobacterium is a thin gram-negative rod and does not form sulfur granules.
- C: Cutibacterium acnes is a skin commensal linked mainly to shunt and implant infections.
- D: Prevotella melaninogenica is a pigmented gram-negative rod without granules.
Question 5 · Easy
Monomicrobial (type II) necrotizing fasciitis in a previously healthy person is most often caused by:
1. AStaphylococcus epidermidis
2. BBacteroides fragilis
3. CStreptococcus pyogenes
4. DClostridium perfringens
Show answerHide answer
Answer: C · Streptococcus pyogenes
Group A streptococcus causes type II necrotizing fasciitis, which can follow minor skin injuries in healthy people. Its streptolysins, hyaluronidase and pyrogenic exotoxins help it spread along fascial planes and can cause streptococcal toxic shock syndrome. Type I necrotizing fasciitis is polymicrobial, with anaerobes, Enterobacterales and streptococci, usually in people with diabetes or after surgery. A tissue Gram stain and urgent surgery are key to management.
- A: S. epidermidis is a low-virulence skin commensal linked to device infections.
- B: B. fragilis is part of polymicrobial (type I) necrotizing infection.
- D: C. perfringens causes gas gangrene, a myonecrosis, usually after deep trauma.
Question 6 · Hard
A man has a swollen, painful knuckle three days after punching someone in the mouth. A slow-growing gram-negative rod from the wound pits the blood agar and smells like bleach; it is oxidase positive, catalase negative and does not grow on MacConkey. The most likely organism is:
1. AAggregatibacter actinomycetemcomitans
2. BEikenella corrodens
3. CPasteurella multocida
4. DCapnocytophaga canimorsus
Answer and explanation in LearnFree account
Question 7 · Medium
Gram stain of a foul-smelling perianal abscess shows many neutrophils and several bacterial morphotypes, but aerobic cultures are negative at 48 hours. The most likely explanation is:
1. AAnaerobic bacteria not recovered aerobically
2. BMycobacteria that do not grow on routine media
3. CToo few organisms present to grow on agar
4. DSkin flora contaminating the slide only
Answer and explanation in LearnFree account
Question 8 · Medium
A man with alcoholic cirrhosis cuts his hand while shucking oysters and within a day has hemorrhagic bullae and sepsis. A halophilic, lactose-positive, oxidase-positive curved gram-negative rod grows from the wound. The organism is most likely:
1. APasteurella multocida
2. BVibrio cholerae
3. CAeromonas hydrophila
4. DVibrio vulnificus
Answer and explanation in LearnFree account
Question 9 · Medium
A 9-year-old develops osteomyelitis of the heel two weeks after stepping on a nail that went through his sneaker. Which organism is most likely?
1. AStreptococcus pneumoniae
2. BClostridium tetani
3. CSalmonella enterica
4. DPseudomonas aeruginosa
Answer and explanation in LearnFree account
Question 10 · Easy
Which organism is the most common cause of acute hematogenous osteomyelitis in both children and adults?
1. ASalmonella enterica
2. BPseudomonas aeruginosa
3. CStreptococcus pyogenes
4. DStaphylococcus aureus
Answer and explanation in LearnFree account
Question 11 · Medium
Large, boxcar-shaped gram-positive bacilli from a gas gangrene wound show a double zone of beta-hemolysis. What is the organism?
1. AListeria monocytogenes
2. BClostridioides difficile
3. CBacillus cereus
4. DClostridium perfringens
Answer and explanation in LearnFree account

Skin, soft tissue and bone

A fish handler has a painful violet swelling on a finger. A biopsy grows a catalase-negative, nonmotile gram-positive bacillus that produces H2S in the TSI butt and is resistant to vancomycin. What is the most likely organism?

Which organism is most often isolated from an infected cat bite?

Yellow 'sulfur granules' in pus from a draining abscess of the jaw suggest infection with:

Monomicrobial (type II) necrotizing fasciitis in a previously healthy person is most often caused by:

A man has a swollen, painful knuckle three days after punching someone in the mouth. A slow-growing gram-negative rod from the wound pits the blood agar and smells like bleach; it is oxidase positive, catalase negative and does not grow on MacConkey. The most likely organism is:

Gram stain of a foul-smelling perianal abscess shows many neutrophils and several bacterial morphotypes, but aerobic cultures are negative at 48 hours. The most likely explanation is:

A man with alcoholic cirrhosis cuts his hand while shucking oysters and within a day has hemorrhagic bullae and sepsis. A halophilic, lactose-positive, oxidase-positive curved gram-negative rod grows from the wound. The organism is most likely:

A 9-year-old develops osteomyelitis of the heel two weeks after stepping on a nail that went through his sneaker. Which organism is most likely?

Which organism is the most common cause of acute hematogenous osteomyelitis in both children and adults?

Large, boxcar-shaped gram-positive bacilli from a gas gangrene wound show a double zone of beta-hemolysis. What is the organism?