MICRO: Skin and MSK Infections

Question 1

3 main mechanisms of disease for skin and MSK

Answer 1

• breach of intact skin e.g. surgery, burns, IV drips
• skin manifestation of systemic infection: salmonella typhi, N. meningitidis, virus (HSV, VZV)
• toxin-mediated skin damage e.g. scarlet fever, toxic shock

Question 2

2 most common bacteria which affect skin and MSK

Answer 2

• strep pyogenes
• staph aureus

Question 3

infections of hair follicles and what bacteria causes this?

Answer 3

• folliculitis
• boils (infection of sebum glands)
• carbuncles
• all S. aureus

Question 4

infections of keratinised epithelium, epidermis, dermis and what pathogen causes these?

Answer 4

• keratin: ringworm/tinea (fungus)
• epidermis: impetigo (S. aureus or S. pyogenes - highly contagious, spread thru direct contact)
• dermis: erysipelas (S. pyogenes: dermal lymphatics > erythema)

Question 5

infections of subcutaneous tissue, fascia, muscle and what pathogen causes it

Answer 5

• subcutaneous tissue: cellulitis (S. aureus or S. pyogenes)
• fascia: necrotising fasciitis (S. pyogenes)
• muscle: myonecrosis (gas) gangrene (C. perfringens)

Question 6

• erythema
• macule
• papule

Answer 6

• erythema: redness due to vasodilation
• macule: circumscribed flat area of altered skin colour
• papule: circumscribed elevated area of skin

Question 7

• vesicle
• bulla
• pustule

Answer 7

• vesicle: small blister of clear fluid
• bulla: large blister of clear fluid
• pustule: small or large blister of pus

Question 8

what pathogen causes surgical wound infections and abscesses?

Answer 8

• S. aureus

Question 9

staphylococcal scalded skin syndrome: Sx and Tx

Answer 9

• Sx: large areas of exfoliating skin + blisters, rarely severely ill
• exfoliative toxins A and B
• Tx: fluid replacement + antibiotics

Question 10

toxic shock syndrome: causes, Sx and Tx

Answer 10

• S. aureus or S. pyogenes exotoxins (superantigens) > cytokine storm
• genital (tampon use) or non-genital sites
• Sx: fever, hypotension, diffuse macular erythematous rash, followed by skin desquamation
• Tx: drainage, antibiotics, fluid replacement

Question 11

scarlet fever cause + Sx

Answer 11

• S. pyogenes exotoxin
• Sx: dotted red rash, strawberry tongue

Question 12

cellulitis cause + Sx

Answer 12

• S. aureus or S. pyogenes > spreads thru subcutaneous tissue
• originates from boils, ulcers or trauma, risk in diabetes
• Sx: red hot swollen lesion, fever, chills

Question 13

necrotising fasciitis pathogen + Tx

Answer 13

• S. pyogenes or S. aureus = flesh eating, invades muscle and adipose, purple skin (emergency)
• Tx: excision of fascia, antibiotics

Question 14

gas gangrene cause + Tx

Answer 14

• C. perfringens (anaerobe)
• spores from soil or animal faeces enter into subcutaneous tissue and deeper muscle = causes necrosis = gas production leading to crepitus
• Tx: immediate surgery to remove tissue, may need amputation

Question 15

acne

Answer 15

• Propionibacterium acnes
• increased sebum production @ puberty + desquamation > blockage of sebaceous glands

Question 16

superficial mycosis of the skin + Sx

Answer 16

• linea versicolor (Malazzesia furfur)
• Sx: hypo or hyper-pigmented macules depending on skin colour + sweating

Question 17

cutaneous mycoses

Answer 17

• dermatophytes (tinea)
• candidiasis: thrush, diaper rash, vaginitis and penile infections

Question 18

varicella-zoster (skin) transmission + reactivation

Answer 18

• inhalation of respiratory droplets or direct contact from skin lesions
• 1st infection: chickenpox, becomes dormant in DRG
• reactivation (shingles): rash, paraesthesia, neuralgia

Question 19

measles Sx

Answer 19

• respiratory Sx, conjunctivitis, Koplik’s spots, maculopapular rash, fever
• lifelong immunity after infection
• much more serious in malnourished kids

Question 20

rubella Sx

Answer 20

• Sx: fever, malaise, irregular maculopapular rash lasting 3 days
• very dangerous for pregnant women: can cause many congenital effects to baby

Question 21

coxsackievirus + echovirus

Answer 21

• hand, foot and mouth
• exanthems (skin rashes e.g. hand/foot), enanthems (lesions on mucosal surfaces e.g. buccal)

Question 22

osteomyelitis

Answer 22

• systemic infection or trauma > bone exposed to pathogen
• M. tuberculosis > chronic infection spreads thru blood
• S. aureus usually acute due to trauma and infection of soft tissue near bone

Question 23

septic arthritis: Sx, pathogens, Tx

Answer 23

• haematogenous route: thru trauma or surgery
• Sx: fever, joint pain, swelling, joint effusion, limited ROM
• pathogens: S. aureus, salmonella, H. influenzae, N. gonorrhoea
• Tx: immediate antibiotics, surgical drainage, wash out

Question 24

Sx, Ix and Tx for osteomyelitis

Answer 24

• Sx: fever, localised pain and tenderness
• Ix: culture of blood, tissue, bone
• Tx: debridement, drainage, antibiotics

Question 25

viral arthritis

Answer 25

• milder, self-limiting
• virus directly infects joints
• e,g, Rubella, chikungunya virus, zika virus

Question 26

reactive arthritis

Answer 26

• occurs when there is a bacterial infection somewhere else in the body
• arthritis occurs due to immune-mediated inflammation

Question 27

which type of skin infection is likely to have polymicrobial infection?

Answer 27

• diabetic foot infection