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USMLE Step 3 (Fach) / Infectious Disease (Lektion)

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• Diphyllobothriasis Pathogen: Diphyllobothrium latum (fish tapeworm – a cestode) Mode of transmission: consumption of raw or undercooked freshwater fish Clinical features:- Usually asymptomatic- Chronic infection leads to vitamin B12 deficiency → anemia  and/or neurological symptoms Diagnostics:- Initial test: complete blood cell (CBC) count may show eosinophilia, megaloblastic anemia- Confirmatory test: stool examination for eggs or proglottids (i.e., segments of adult tapeworms) Treatment: praziquantel
• Lymphatic filariasis Pathogens:- Wuchereria bancrofti: a nematode; responsible for most cases of lymphatic filariasis worldwide- Brugia malayi and Brugia timori: found in Asia Mode of transmission: female mosquito bite (Aedes, Mansonia, Anopheles, and Culex) Incubation period: 9-12 months Life cycle: Mosquito introduces filarial larvae into host via bite wound → Larvae mature into adult worms that reside in the lymphatic system → Adult worms produce microfilariae → Microfilariae move throughout vascular and lymphatic system → Microfilariae are consumed by a female fly during a blood meal → Microfilariae mature into larvae, thus completing the cycle. Clinical features:- Fever- Painful lymphadenopathy (due to worms invading lymph nodes, causing inflammation) and retrograde lymphangitis → lymphedema with disfiguration of the lower extremities (elephantiasis)- Hydrocele Diagnostics:- Blood smear obtained at night (and staining with Giemsa or hematoxylin and eosin): detection of microfilariae  - Serology: elevated levels of antifilarial IgG4 Treatment:- Diethylcarbamazine (DEC): drug of choice- Ivermectin: used in areas where onchocerciasis is prevalent, as DEC can worsen onchocercal eye disease- Elevation of the affected extremity, exercise, and wearing therapeutic shoes: recommended for those with lymphedema- Surgery for hydrocele
• Noncholera Vibrio infection Pathogen:- Vibrio parahaemolyticus: non-lactose fermenter, gram-negative bacilli- Vibrio vulnificus: lactose fermenter, gram-negative bacilli Transmission:- Foodborne (raw or undercooked shellfish)- Wounds infected by contaminated sea water Incubation period: 12-52 hours Clinical features:- Inflammatory diarrhea- Low-grade fever, vomiting, abdominal pain- Cellulitis, bullous skin lesions Treatment: in severe infection or wound infection, doxycycline or fluoroquinolone (e.g., ciprofloxacin) Complications:- Complications of noncholera Vibrio infection are common in patients with high levels of free iron (e.g., liver disease, hemochromatosis) or immunocompromise.- Septic shock and necrotizing fasciitis associated with Vibrio vulnificus infection (rare)
• Common diagnostic tests for TB Tuberculin skin test:- High specificity in non-BCG-vaccinated patients- Cost effective, requires training for interpretation- Can diagnose latent but not active TB- Return visit required in 48 hours for final result Interferon-gamma release assay:- Blood draw required, 1 patient visit- High specificity with results in 16-24 hours- No interference with BCG vaccine- Cannot differentiate between latent & active TB Smear microscopy:- Cost-effective, easy to obtain, low sensitivity- Estimates quantitative number of bacilli- Can determine active infetion, monitor treatment progress- Cannot distinguish from non-TB mycobacteria Sputum culture:- Gold standard, quantitative results, takes 3-8 weeks- Allows for drug sensitivity testing Nucleic acid amplification:- High specificity, higher sensitivity than smear microscopy- Diagnosis in 1-2 days- Can differentiate from non-TB mycobacteria- Can be positive in patients after TB treatment
• Respiratory diphtheria Incubation period: 2-5 days The patient presents initially with prodromal symptoms: fever, malaise, and sore throat 4-5 days after the onset of prodromal symptoms, symptoms due to the local and systemic effects of the toxin occur- Anterior nasal diphtheria: bloody rhinorrhea- Tonsillar and pharyngeal diphtheria→  A grayish white pseudomembrane over the posterior pharyngeal wall, and/or tonsils→ Any attempt to scrape off the pseudomembrane exposes the underlying capillaries and results in heavy bleeding. - Bull neck, which may result in airway obstruction  - Foul mouth odor- Laryngeal diphtheria: difficulty breathing, inspiratory stridor   Systemic features- Myocarditis → arrhythmias- Reversible polyneuropathy Diagnostics:- Cultures of pharyngeal swabs are used to confirm the diagnosis  → Microscopic examination of pharyngeal swabs or culture isolates reveals multiple C. diphtheriae clustered together (Classically described as a Chinese letter pattern)  → Culture media of choice are cystine-tellurite agar or Löffler medium- Once the culture reveals C. diphtheriae, one or both of the following tests are used to identify whether the strain is toxigenic:→ Elek's test  → Polymerase chain reaction to identify the tox gene- Test for myocarditis: Conduct multiple ECGs and serial measurement of cardiac markers. ECG typically reveals QT prolongation, ST-T wave changes, and/or first-degree heart block.  Treatment:- The patient should be isolated as soon as diphtheria is suspected.  - Antibiotic therapy : IM injections of Penicillin G or oral/IV erythromycin for 14 days- Immediate administration of diphtheria antitoxin: The antitoxin can only neutralize unbound toxin and should therefore be administered early in the course of the disease.  - Airway support  
• Sporotrichosis Pathogen: Sporothrix schenckii Risk factor: traumatic injury to the skin while gardening (e.g., thorn prick) Clinical features:- Pustules and ulcers with ascending lymphangitis- Disseminated disease: pneumonia, meningitis Diagnostics:- Fungal culture (sputum, pus, biopsy tissue) showing dimorphic, cigar-shaped yeast- Septate hyphae, rosette-like clusters of conidia at the tips of the conidiophores Treatment:- Oral itraconazole or potassium iodide (3-6 months)- Disseminated: IV amphotericin B
• Acute cystitis and asymptomatic bacteriuria during pregnancy Asymptomatic bacteriuria:- Positive urine cultures (≥100,000 colonies/mL) in asymptomatic patient- Screening at initial prenatal visit- Treatment ↓ progression to UTI & complications (e.g., preterm birth) Acute cystitis:- Symptomatic patient with positive urine culture- Complicated UTI Treatment:- Cephalexin- Amoxicillin-clavulanate- Fosfomycin
• Persons needing antimicrobial prophylaxis following Neisseria meningitidis exposure - Household members- Roommates or intimate contacts- Child care center workers- Persons directly exposed to respiratory or oral secretions (e.g., kissing, mouth-to-mouth resuscitation, endotracheal intubation/management)- Person seated next to an affected person for ≥8 hours (e.g., airline traveler) Treatment:- Rifampin (600 mg orally twice for 2 days)- Ciprofloxacin (500 mg single dose)- Ceftriaxone (250 mg single dose)
• Catheter-related bloodstream infection (CRBSI) Pathogen:- Coagulase-negative staphylococci - Staphylococcus aureus- Gram-negative bacilli Initial work-up:1. 2 sets of blood culture (ideally one from a peripheral site and one from the central catheter)2. Empiric antibiotic therapy with vancomycin plus cefepime (or gentamicin) Catheter removal is indicated if any of the following are present:- Severe sepsis- Hemodynamic instability- Evidence of metastatic infection (e.g., endocarditis)- Pus at the exit site of the catheter- Continued symptoms after 72 hours of empiric antibiotics
• Bite wounds with a high risk of infection - Crush injuries- Bites on hands or feet- Wounds on body > 12 hours or on face > 24 hours- Cat bites (except on face)- Human bites (except on face)- Bite wounds in immunocompromised hosts Prophylaxis: Amoxicillin-clavulanate
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• Cat-scratch disease Benign, self-limiting infectious disease that is transmitted mainly by cats (via scratching, biting, or licking)   Pathogen: Bartonella henselae (gram-negative bacillus, aerobic)   Epidemiology: predominantly affects children and adolescents Clinical features:- General: malaise, loss of appetite, fever- One or more 5-10 mm large, erythematous, non-tender cutaneous papules or vesicles develop approx. 4-6 days after exposure (in > 50% of cases) at the site of inoculation.- Swollen, tender lymph nodes 1-3 weeks following exposure  → Develops as primary lesions disappear→ Usually unilateral, occasionally suppurative→ Most commonly involves lymph nodes of axillae, neck, or groin (nearest the site of inoculation)→ Resolves after 2-4 months  - In immunocompromised individuals  → Bacillary angiomatosis  → Hepatic peliosis  → Bacteremia and endocarditis Diagnostics:- Bacterial culture from blood, swabs, or lymph node aspirate- Antibody testing: Indirect fluorescence assay (IFA) testing and enzyme-linked immunoassay (ELISA)- Histological study:→ Warthin-Starry staining of the involved lymph node may show clusters of rod-shaped bacteria.→ H&E staining of cutaneous lesions may show necrotizing granuloma formation. Treatment:- Mild or moderate cases: azithromycin (5-day course) to decrease lymphadenopathy and the duration of illness  - Persistent and/or disseminated cases (e.g., bacillary angiomatosis): erythromycin (or doxycycline); in case of CNS involvement or endocarditis, rifampicin should be used in combination with one of these drugs.
• Dengue Distribution: tropical regions worldwide, particularly Asia (e.g., Thailand)Most common viral disease affecting tourists in tropical regions Vector-borne: mosquitoes most commonly from the species Aedes aegypti  These mosquitoes can also transmit chikungunya, Zika virus, and yellow fever. Classic dengue fever:- Incubation period: 2-14 days- Children are usually asymptomatic- Starts with fever and malaise that lasts ∼1 week- Severe arthralgia and myalgia (often referred to as “break-bone fever”)- Severe headache and retro-orbital pain- Maculopapular, measles-like exanthem (typically appears 2-5 days following fever)- Generalized lymphadenopathy Dengue hemorrhagic fever (DHF):- Occurs in 1-2% of cases- Generally develops as the initial fever subsides (∼1 week after onset)- Temperature change: ranges from hypothermia to a second spike in fever- Abdominal pain, vomiting- Changes in mental status (e.g., confusion)- Hemorrhagic manifestations (e.g., petechiae, epistaxis, gingival bleeding)  → Positive tourniquet test- Increased vascular permeability → signs of pleural effusion and/or ascites- Dengue shock syndrome (DSS): DHF + shockDengue hemorrhagic fever is more frequent in individuals who experience a repeat infection with a second serotype, especially serotype 2! Diagnostics:Laboratory tests: Leukopenia, Thrombocytopenia, ↑ AST, Hct elevated ≥ 20% of normal values if vascular permeability (in DHF)- Best test for confirming infection: serology (IgM, IgG)- Alternatives: PCR, Molecular methods (ELISA) Treatment: Symptomatic- Fluid administration to avoid dehydration- Acetaminophen  - DHF→ Blood transfusions in case of significant internal bleeding (e.g., epistaxis, gastrointestinal bleeding, or menorrhagia)→ IV fluids
• Prevention & control of Clostridium difficile Infection control:- Monitor infection rates in hospital- Contact isolation- Wash hands with soap & water- Disinfect or use disposable stethoscopes Hospital environment:- Clean patient room & environment with effective agent (e.g., hydrochlorite solution)- Consider cleaning medical equipment (e.g., blood pressure cuffs) Antibiotic restriction:- Possibly restrict use of clindamycin, fluoroquinolones & cephalosporins- Consider using aminoglycosides or trimethoprim-sulfamethoxazole instead of fluoroquinolones
• Chlamydia in pregnancy Screening:- Universal screening at first prenatal visit- High risk: repeat screening in 3rd trimester Risk factors:- Age < 25 years- History of STI- Recent new partner- History of multiple partners- History of unprotected intercourse Obstetric complications:- Preterm premature rupture of membranes- Preterm labor- Postpartum endometritis Fetal complications:- Neonatal conjunctivitis- Neonatal pneumonia Treatment: Azithromycin
• Management of uncomplicated pediatric pneumonia Clinical features:- Fever- Tachypnea- Cough- Adventitious lung sounds Preschool age OR focal lung finding → Streptococcus pneumoniae → High-dose Amoxicillin Older child OR well-appearing with bilateral lung findings → Mycoplasma pneumoniae → Azithromycin
• Contraindications to varicella-zoster virus vaccine - Anaphylaxis to neomycin - Anaphylaxis to gelatin - Pregnancy - Immunodeficient state→ Congenital immunodeficiencies→ Long-term immunosuppresive therapy→ Hematologic or solid tumors→ Severe HIV infection
• Human bites Microbiology:- Eikenella corrodens- α-hemolytic streptococci (viridans groups streptococci)- Staphyloccocus aureus- Other anaerobes (e.g., Fusobacterium, Prevotella) Management:- Local wound care & irrigation- No primary closure (except face)- Amoxicillin‑clavulanate or cephalosporin of 2nd or 3rd generation- Consider tetanus boost
• Epiglottitis Peak incidence: 6-12 years (but can occur in any age, including adults, especially when unimmunized) Pathogens:- Traditionally: Haemophilus influenzae type b (Hib)- Most cases now involve: Streptococcus pyogenes, Streptococcus pneumoniae, Staphylococcus aureus, Non-typable H. influenzae Clinical features:- Acute onset of high fever (39-40 °C)- Toxic appearance- “Tripod” position  - Sore throat- Dysphagia- Drooling- Muffled voice (i.e., resembling a “hot-potato” voice) with painful speech- Respiratory distress (inspiratory retractions, cyanosis) and inspiratory stridor  - Restlessness and/or anxiety Diagnostics: Primarily clinicalIf there are definitive signs of epiglottitis present → airway management before attempting to visualize the epiglottis and supraglottic structures! Treatment:1. Keep child comfortable and as calm as possible2. If signs of respiratory distress → emergency airway management- Extubation should be performed 2-3 days (at the earliest) after starting antibiotic treatment.3. IV Antibiotics- Empiric therapy: third-generation cephalosporin (e.g., ceftriaxone) + antistaphylococcal antibiotic against MRSA (e.g. clindamycin), if MRSA is suspected4. IV fluid resuscitation if required
• Serum sickness Etiology:- Antivenom or antitoxin containing animal proteins or serum (→ “serum” sickness)  - Medications, most frequently antibiotics (e.g., penicillin, amoxicillin, cefaclor, trimethoprim-sulfamethoxazole)- Infections: Hepatitis B virus Clinical features:- Fever- Rash (urticarial or purpuric)- Arthralgias, myalgia- Headache, blurred vision- Abdominal pain, diarrhea, nausea/vomiting- Lymphadenopathy Course:- Symptoms appear 1-2 weeks following initial exposure.  - They resolve within a few weeks of discontinuation. Treatment: stop the offending agent
• Rabies prophylaxis guidelines Pre-exposure prophylaxis:- Rabies vaccine on days 0, 7 & 21 or 28 Post-exposure prophylaxis, previously unvaccinated:- Rabies immunoglobulin on day 0- Rabies vaccine on day 0, 3, 7 & 14 Post-exposure prophylaxis, previously vaccinated:- Rabies vaccine on days 0, 3
• Rabies Transmission:- Spread through saliva of rabid animal after bite injury- Most common animal reservoir worldwide: dogs  - Most common animal reservoirs in the US: bats, raccoons, skunks, and foxes Clinical features:Rabies is almost always fatal once the symptoms have appeared!- Incubation period: 1-12 weeks average- Flu-like symptoms- Locally: pain, paresthesia, and pruritus near the bite site   Encephalitic rabies (most common type):- Hydrophobia  - Anxiety, agitation and combativeness alternating with calm periods- Confusion and hallucinations- Fasciculations- Seizures- ↑ Muscle tone and reflexes with nuchal rigidity- Autonomic symptoms: e.g., hypersalivation- Coma and death within days to weeks of the development of neurological symptoms   Paralytic rabies (< 20% of cases):- Flaccid paralysis, gradually ascending and spreading from bite wound  - Paraplegia and loss of sphincter tone- Respiratory failure and death  
• Staphylococcal toxic shock syndrome Etiology- ∼ 50% of cases: caused by prolonged use of tampons over several days- Postsurgical or after wound infections Pathophysiology: exotoxins (see resistance and virulence of staphylococci) Clinical findings: if tampon related, symptoms usually appear within five days after menstruation.- Widespread exanthem with skin peeling involving palms and soles- Myalgias, vomiting, diarrhea- Symptoms of shock: hypotension, tachycardia- Fever- Altered mental status: confusion, lethargy, coma Laboratory tests: ↓ Platelets, ↑ Bilirubin, ↑ transaminase Treatment:- IV fluids- Remove focus of infection (e.g., drain surgical wounds, remove tampon)- Antibiotics→ First-line antibiotics against MSSA TSS: clindamycin + oxacillin→ First-line antibiotics against MRSA TSS: clindamycin + vancomycin
• Pediatric septic arthritis Age <3 months: Staphylococcus aureus, group B Streptococcus, gram-negative bacilliAge ≥3 months: Staphylococcus aureus, group A Streptococcus Clinical features:- Acute-onset joint pain, swelling, limited motion- Refusal to bear weight- Fever ≥ 38.5 C (101.3 F) Diagnosis:- ↑ WBC, ESR, CRP- Blood culture- Joint aspiration (synovial WBC count of >50,000/mm3)- Effusion on ultrasound/MRI Treatment:- Joint drainage & debridement- IV antibiotics 
• Chikungunya fever Epidemiology:- Tropical/subtropical parts of Central/South America, Africa & Asia- Vector: Aedes mosquito (also transmits dengue & Zika) Manifestations:- Incubation period: 3-7 days- High fevers & severe polyarthralgias (almost always present)- Headache, myalgias, conjunctivitis, maculopapular rash- Lymphopenia, thrombocytopenia, transaminitis Management:- Supportive care (initial symptoms usually resolve in 7-10 days)- Chronic arthralgias/arthritis occurs in > 50% (may require methotrexate)
• Treatment options for latent tuberculosis infection - Isoniazid & rifapentine weekly for 3 months under direct observation(not recommended in patients with HIV) - Isoniazid monotherapy for 6-9 months - Rifampin for 4 months
• Scombroid poisoning A foodborne illness caused by ingestion of histidine-rich fish that has spoiled or been inadequately refrigerated. Histidine is converted to histamine by bacteria at warm temperatures. Typically manifests with flushing, urticaria, nausea, edema, diarrhea, and abdominal cramps within minutes to hours after ingestion.
• Actinomycosis Pathogen:- Actinomyces are primarily anaerobic, gram-positive, non-acid fast, branching, rod-shaped bacteria.- Actinomyces bacteria, particularly Actinomyces israelii, are found in the normal oral flora. Predisposing factors:- Poor dental hygiene, oral surgery (e.g., tooth extraction), local tissue inflammation (e.g., tonsillitis, tumor), comorbidities (e.g., diabetes) → cervicofacial actinomycosis- Intestinal surgery, foreign body ingestion, tumor → abdominal actinomycosis- Ascending infection from the uterus, associated with intra-uterine contraceptive devices → pelvic actinomycosis- History of aspiration, recent oral infection, or ongoing pulmonary malignancy (see aspiration pneumonia) → thoracic actinomycosis Cervicofacial actinomycosis (most frequent form of actinomycosis)- Slowly progressive mass in the neck and/or face; most commonly in the mandible region- Usually painless nodular lesions- Becomes indurated with purulent discharge that contains sulfur granules from fistulae and draining sinus tracts. Abdominal and pelvic actinomycosis- Fever, abdominal discomfort, changes in bowel habits- Possible pathological vaginal bleeding or discharge Thoracic actinomycosis- Cough, chest pain- Possible hemoptysis with yellow granules- Constitutional symptoms: fatigue, weight loss, malaise Treatment: Penicillin (drug of choice)- Alternatives (in the event of penicillin allergy): doxycycline, clindamycin
• Mucormycosis Definition: rare fungal infection, primarily affecting immunocompromised patients and patients with diabetes mellitus Etiology: fungi of the order Mucorales, most commonly Rhizopus oryzae; ubiquitous fungi found in vegetation and soil Risk factors:- Diabetes mellitus, particularly type 1 (ketoacidosis)- Immunosuppression: hematologic malignancies, stem cell or solid organ transplantation, treatment with glucocorticoids, AIDS- Iron overload or treatment with deferoxamine Symptoms and clinical findings:- Rhino-orbital-cerebral mucormycosis: the most common form; presents with sinusitis, fever, headache  - Pulmonary mucormycosis: aggressive infection of the bronchioli and alveoli, presenting with fever and hemoptysis- Disseminated mucormycosis: (rare) invasion of the vasculature with subsequent dissemination; may affect any organ Diagnostics:- Biopsy: broad, nonseptate hyphae, with right-angle branching on microscopy  - Imaging: assess the extent of tissue damage and organ involvement Treatment:- Antifungal treatment: amphotericin B, step-down therapy with oral azole  - Surgical debridement- Elimination of risk factors, aggressive blood glucose control Complications: palatal eschars, cerebritis, mediastinitis, cardiac involvement
• Clostridium difficile – Disease severity Nonsevere: leukocytosis < 15,000/mm3 and serum creatinine < 1.5 of baseline Severe: leukocytosis ≥ 15,000/mm3 OR serum creatinine ≥ 1.5 of baseline Fulminant: decreased blood pressure, shock, ileus or toxic megacolon
• Mycobacterium fortuitum Rapidly growing, nontuberculous mycobacteria (NTM) - Produces chronic, nonhealing wounds on skin, soft tissue, surgical sites, and occasionally on prosthetic devices. Diagnosis: Biopsy of deep wound
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• CLABSI – Treatment Duration:- Uncomplicated non–S. aureus CLABSI: 7 to 14 days - S. aureus CLABSI: at least 4 weeks - Bacteremia clearance should be confirmed with follow-up blood cultures
• Travelers' diarrhea – Prevention Prophylaxis in patients with:- Inflammatory bowel disease- Immunocompromise (HIV, organ transplant, immunosuppressant drugs) Drug of choice: Ciprofloxacin
• Cyclospora Protozoan infection Etiology: - Consumption of fecal-contaminated food or water- Consumption of fresh produce imported from tropical areas Epidemiology: Peru, Guatemala, Haiti, and Nepal Clinical features:- The incubation period is approximately 1 week (range, 2 days to ≥2 weeks)- Crampy abdominal pain, anorexia, bloating, decreased appetite, fatigue, flatulence, low-grade fever, malaise, nausea, watery diarrhea, and weight loss Diagnostics:- Microscopy: oocysts with modified acid-fast staining- Fluorescence microscopy- Several stool specimens- Polymerase chain reaction assays appear to have the greatest sensitivity for the diagnosis of a Cyclospora infection. Treatment: one double-strength tablet of trimethoprim-sulfamethoxazole taken orally twice daily for 7 to 10 days
• Posaconazole Antifungal prophylactic therapy
• Posaconazole Antifungal prophylactic therapy
• Purulent infections (furuncle, carbuncle, abscess) – Treatment Mild:- Incision & drainage Moderate:1. Incision & drainage2. Empiric antibiotic treatment: TMP/SMX or doxycycline Severe:1. Incision and drainage  2. Empiric antibiotic treatment covering MRSA: vancomycin or daptomycin or linezolid
• Nonpurulent infections (erysipelas, cellulitis, necrotizing fasciitis) Mild:- Oral antibiotics: penicillin V or cephalosporin or dicloxacillin or clindamycin Moderate:- IV antibiotics: penicillin or ceftriaxone or cefazolin or clindamycin Severe:- Surgical debridement (with culture and sensitivity testing)- Empiric antibiotic treatment: vancomycin PLUS piperacillin/tazobactam 

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