Skin, Soft Tissue & Bites
Core idea
- Gram-positive cocci dominate skin infections
- Water, animal, or soil exposure points to unusual organisms
| Lesion | Clue | Organism |
|---|---|---|
| Furuncle / carbuncle | Hair follicle abscess; neck, axilla, buttock | S. aureus |
| Hot-tub folliculitis | Follicular rash after pool or spa | P. aeruginosa |
| Acne vulgaris | Follicle plus sebaceous inflammation | Cutibacterium (Propionibacterium) acnes |
| Nonbullous impetigo | Honey-colored crusts; catalase negative | S. pyogenes |
| Bullous impetigo | Flaccid bullae; catalase positive | S. aureus (exfoliative toxin) |
| Erysipelas | Raised, sharply demarcated, facial, rapid | S. pyogenes |
| Cellulitis | Warm, red, spreading skin | S. aureus, S. pyogenes |
| Green pus wound | Burns; sweet odor | P. aeruginosa |
| Ecthyma gangrenosum | Necrotic skin lesion; neutropenia | P. aeruginosa bacteremia |
| Cutaneous anthrax | Painless papule → black eschar | B. anthracis |
| Fish-tank granuloma | Aquarium handling; slow nodule | M. marinum |
| Seawater / oyster cellulitis | Bullae; liver disease or iron overload | V. vulnificus |
| Lymphocutaneous nodules | Thorn prick; gardeners | Sporothrix schenckii |
| Jaw swelling, draining sinus | Dental extraction or decay; sulfur granules | Actinomyces israelii |
| Mycetoma | Swelling, sinuses, granules in discharge | Nocardia, Actinomyces, Madurella, Pseudallescheria |
| Vesicles after nerve pain | Dermatomal or grouped | HSV, VZV |
Wounds
- Clean surgical wound
- S. aureus
- Dirty surgical wound
- S. aureus, enteric gram-negatives, anaerobes
- Trauma or soil
- Clostridium, enterics, Pseudomonas
- Puncture through footwear
- Pseudomonas
Bites & scratches
| Source | Organism |
|---|---|
| Cat or dog (general) | Pasteurella multocida |
| Dog; asplenic host | Capnocytophaga canimorsus |
| Human bite / clenched fist | Eikenella corrodens |
| Rat | Streptobacillus moniliformis, Spirillum minus |
| Cat scratch + nodes with stellate granulomas | Bartonella henselae |
Exam trap
- Catalase status separates the two impetigo causes
Ear, Throat, Mouth & Eye
Ear, sinus, mouth
| Condition | Clue | Organisms |
|---|---|---|
| Acute otitis media | Bulging red drum; fever; pain relief if drum perforates | S. pneumoniae, nontypeable H. influenzae, M. catarrhalis, RSV, rhinovirus |
| Sinusitis | Facial pain, low fever | Same as otitis media |
| Otitis externa | Swimmer's ear; mixed flora | Pseudomonas, Proteus, S. aureus, Candida |
| Malignant otitis externa | Severe pain; diabetic; skull-base spread | P. aeruginosa |
| Painful gums, necrosis | Overgrowth of resident oral anaerobes | Fusobacterium + spirochetes |
| Oral thrush | White plaques over red base | Candida (infants, HIV, steroids, antibiotics) |
| Common cold | Rhinitis, sneeze, cough | Rhinovirus (most), coronavirus, adenovirus, metapneumovirus |
Pharyngitis
| Clue | Cause |
|---|---|
| Tonsillar exudate, tender nodes, sandpaper rash | S. pyogenes (erythrogenic toxin) |
| Vesicles / papules on posterior palate | Coxsackie A |
| Fatigue, nodes, atypical lymphocytes, heterophile positive | EBV |
| Gray adherent membrane, bull neck, bleeds when peeled | C. diphtheriae |
Diphtheria mechanism
Toxin-carrying phage→Toxin inactivates EF-2→Protein synthesis halts→Membrane, myocarditis, neuropathy
Eye
| Clue | Cause |
|---|---|
| Periorbital swelling, eosinophilia, myalgia | Trichinella |
| Hordeolum | S. aureus |
| Unilateral eyelid swelling at bite site; Latin America | Trypanosoma cruzi |
| Neonatal conjunctivitis, first week | N. gonorrhoeae (days 2–5) |
| Neonatal conjunctivitis, later | C. trachomatis D–K (about 5–14 days) |
| Watery, photophobic, epidemic | Adenovirus |
| Purulent adult/child | S. aureus, S. pneumoniae, H. influenzae |
| Scarring, inturned lashes, blindness | C. trachomatis A–C |
| Chorioretinitis (neonate, AIDS) | Toxoplasma, CMV |
| Keratitis in infant of drug-using mother | Congenital syphilis |
Heart: Endocarditis & Carditis
AcuteRapid, destructive
- Abrupt fever, rigors, new murmur
- Janeway lesions, septic emboli
- Usual cause: S. aureus
- Injection drug use → tricuspid valve
SubacuteIndolent
- Weeks of low fever, fatigue, weight loss
- Splinter hemorrhages, emboli
- Damaged or abnormal valve
| Setting | Organism |
|---|---|
| Native valve after dental work | Viridans streptococci (about half) |
| Culture-negative, oral flora gram-negatives | Haemophilus aphrophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella |
| Urinary, biliary, or prostate procedure | Enterococcus faecalis |
| Colon neoplasm association | Streptococcus gallolyticus (bovis) |
| Prosthetic valve or IV line | S. epidermidis |
| Drug use; fungal | Candida (pseudohyphae), Aspergillus, Pseudomonas |
Myocarditis / cardiomyopathy
- Viral: coxsackie B, parvovirus B19, adenovirus
- Parasitic: T. cruzi (Chagas; dilated cardiomyopathy)
- Bacterial: Borrelia (conduction block), C. diphtheriae (toxin)
- Rheumatic fever (post-S. pyogenes) → pancarditis
Pitfall
- Rheumatic carditis is immune-mediated, not a direct infection
Lower Respiratory Tract
| Syndrome | Clue | Organism |
|---|---|---|
| Epiglottitis | Drooling, unvaccinated toddler | H. influenzae b |
| Croup | Barking cough, stridor | Parainfluenza |
| Bronchiolitis | Wheezing infant | RSV |
| Prolonged cough | Paroxysms, lymphocytosis | Bordetella pertussis |
| Measles pneumonia | Giant cells; malnutrition | Measles virus |
Typical pneumonia (lobar, productive)
| Clue | Organism |
|---|---|
| Rust-colored sputum; most common adult cause | S. pneumoniae |
| Currant-jelly sputum, alcoholism, abscess | Klebsiella pneumoniae |
| Post-influenza, ventilated, abscess | S. aureus |
| Cystic fibrosis, neutropenia, burns, CGD | Pseudomonas |
| Foul sputum, aspiration | Mixed anaerobes |
Atypical pneumonia (dry cough, patchy)
| Clue | Organism |
|---|---|
| Young adult, school-age child | Mycoplasma pneumoniae |
| Water systems, smoker, GI + neuro signs, low sodium | Legionella (charcoal-yeast agar) |
| Bird contact | Chlamydophila psittaci |
| AIDS, ground-glass, hypoxia, silver-stain cysts | Pneumocystis jirovecii |
| Travel to East Asia | SARS coronavirus |
| Rodent exposure, southwestern US | Hantavirus |
Chronic cough, weight loss, night sweats
| Exposure | Organism |
|---|---|
| Older, HIV, immigrant | M. tuberculosis |
| Bird or bat droppings; yeast in macrophages | Histoplasma |
| Desert dust, southwestern US | Coccidioides |
| Decaying wood, eastern/central US | Blastomyces |
GI Tract & Liver
Watery diarrhea
- Toxin or viral small-bowel process
- No leukocytes, little fever
Dysentery
- Invasive colon disease
- Blood, pus, cramps, tenesmus
Preformed toxin (early vomiting)
| Source | Onset | Organism |
|---|---|---|
| Ham, potato salad, pastries | 1–6 h | S. aureus (stable enterotoxin) |
| Reheated rice | under 6 h | B. cereus emetic |
| Meat, vegetables | over 6 h | B. cereus diarrheal; C. perfringens |
Noninflammatory
| Organism | Key point |
|---|---|
| Rotavirus | Infants; villus blunting |
| Norovirus | Cruise ships, adults |
| Vibrio cholerae | Rice-water stool; cAMP rise via adenylate cyclase toxin |
| ETEC | Traveler's; LT raises cAMP, ST raises cGMP |
| EHEC | Undercooked beef; Shiga-like toxin; bloody, afebrile |
| Giardia | Campers; greasy stool; sucking disk |
| Cryptosporidium | Acid-fast oocysts; severe in AIDS |
Inflammatory
| Organism | Key point |
|---|---|
| Campylobacter jejuni | Poultry; later Guillain-Barré, reactive arthritis |
| Salmonella | Eggs, poultry; osteomyelitis in sickle cell |
| Shigella | Very low infectious dose; no animal reservoir |
| Yersinia enterocolitica | Right-sided pain mimics appendicitis; grows cold |
| C. difficile | After antibiotics; pseudomembranes |
| Entamoeba histolytica | Flask-shaped ulcers; liver abscess |
Viral hepatitis
| Virus | Features |
|---|---|
| A | Fecal-oral; naked RNA; never chronic |
| B | Blood, perinatal; DNA; chronic → cirrhosis, HCC |
| C | Blood; enveloped RNA; most become chronic |
| E | Fecal-oral; lethal in pregnancy |
Other abdominal
- Biliary obstruction: Ascaris, Fasciola
- Periportal fibrosis: Schistosoma mansoni
- Peritonitis: Bacteroides fragilis plus E. coli
- Pancreatitis with parotid swelling: mumps
Urinary & Genital Infections
| Syndrome | Clue | Organism |
|---|---|---|
| Cystitis | Dysuria, frequency | E. coli (most), Proteus, Klebsiella |
| Young sexually active woman | Gram-positive coccus, novobiocin resistant | S. saprophyticus |
| Pyelonephritis | Flank pain, high fever | E. coli |
| Urethritis | Gram-negative diplococci in neutrophils | N. gonorrhoeae |
| Urethritis, culture negative | Intracellular inclusions | C. trachomatis |
| Urethritis, no cell wall | Urease positive | Ureaplasma |
| Bacterial vaginosis | pH over 4.5, fishy odor, clue cells | Gardnerella + anaerobes |
| Candidal vulvovaginitis | Cottage-cheese discharge | Candida |
| Trichomoniasis | Frothy discharge; motile trophozoites | T. vaginalis |
| Pelvic inflammatory disease | Cervical motion tenderness | Gonococcus, chlamydia, mixed |
Genital lesions
| Lesion | Cause |
|---|---|
| Warts | HPV (also molluscum) |
| Painful grouped vesicles, recurrent | HSV |
| Painless indurated ulcer | T. pallidum |
| Painful soft ulcer, suppurative node | H. ducreyi |
| Painless beefy ulcer; Donovan bodies | Klebsiella granulomatis |
| Transient papule → matted nodes, fistulas | C. trachomatis L1–L3 |
| Scrotal or limb elephantiasis | Wuchereria, Brugia |
Central Nervous System
Bacterial meningitis by host
| Host | Organism |
|---|---|
| Newborn to 2 months | Group B strep, E. coli, Listeria |
| Unvaccinated young child | H. influenzae b |
| Teen / young adult, petechiae | N. meningitidis |
| Adult, vaccinated child, elderly | S. pneumoniae |
| Transplant, cancer, pregnancy | Listeria monocytogenes |
| AIDS | Cryptococcus neoformans (encapsulated yeast) |
| Cranial palsies, low CSF glucose | M. tuberculosis |
CSF pattern
| Cause | Cells | Glucose | Protein |
|---|---|---|---|
| Normal | 0–5 lymphocytes | 40–70 | under 40 |
| Viral | Mildly high, lymphocytes | Normal | Normal / mild up |
| Bacterial | Very high, neutrophils; cloudy | Low | High |
| Fungal / TB | Moderate, lymphocytes | Low | Moderate–high |
Parenchymal & other
- Primary amebic meningoencephalitis
- Warm freshwater; olfactory route; Naegleria
- Encephalitis
- HSV-1: temporal lobe, RBCs in CSF
- Summer mosquito-borne: West Nile, St. Louis, equine viruses
- Brain abscess
- Streptococci and Bacteroides; avoid lumbar puncture
- Neurocysticercosis: seizures in endemic-area adults; Taenia solium
- Reye syndrome: aspirin after influenza or varicella
- Facial palsy after bull's-eye rash: Borrelia
- Ascending paralysis: Campylobacter, influenza
- AIDS lesions: Toxoplasma
Safety
- Suspected mass lesion needs imaging before lumbar puncture
Blood, Bone, Joint & Rash
Blood findings
| Finding | Cause |
|---|---|
| Megaloblastic anemia | Diphyllobothrium latum (B12) |
| Microcytic anemia | Hookworm, whipworm |
| Cyclic fever, rings in RBC | Plasmodium |
| Asplenic, New England, ticks | Babesia microti |
| Neutrophilia | Extracellular bacteria |
| Eosinophilia | Allergy; migrating helminths |
| Atypical lymphocytes, heterophile positive | EBV |
| Same, heterophile negative | CMV, Toxoplasma |
| Marked lymphocytosis, cough | Pertussis |
Bone & joint
| Setting | Organism |
|---|---|
| Osteomyelitis, usual | S. aureus |
| Neonate | S. aureus, group B strep, gram-negative rods |
| Sickle cell disease | Salmonella |
| Penetrating foot trauma | Pseudomonas |
| Septic arthritis, general | S. aureus |
| Sexually active adult | N. gonorrhoeae |
| Prosthetic joint | Coagulase-negative staphylococci |
| Chronic monoarthritis | M. tuberculosis, fungi |
| Large joint after tick bite | Borrelia |
| Reactive after gut or genital infection | Salmonella, Shigella, Campylobacter, Yersinia, Chlamydia |
Rash patterns
| Rash | Disease / cause |
|---|---|
| Sandpaper, trunk → limbs | Scarlet fever; S. pyogenes toxin |
| Sunburn-like, palm peeling, shock | Toxic shock; TSST-1 |
| Perioral, bullae, sheet-like peeling | Scalded skin; exfoliative toxin |
| Wrists/ankles → palms and soles, petechiae | Rickettsia rickettsii |
| Trunk → limbs, spares palms and soles | Epidemic typhus |
| Palms and soles, copper macules | Secondary syphilis |
| Head → body, Koplik spots | Measles |
| Expanding target lesion | Lyme disease |
| Rapid petechiae and shock | Meningococcemia |