Common Infections.

Upper Respiratory Tract Infections (URTIs)

Acute Otitis Media (AOM)

Streptococcal Pharyngitis ("Strep Throat")

Acute Bacterial Rhinosinusitis (ABRS)

Acute Otitis Media (AOM)

• Pathogens: Streptococcus pneumoniae, non-typeable Haemophilus influenzae, Moraxella catarrhalis.

• Presentation: Otalgia, bulging tympanic membrane, fever, decreased hearing.

• 1st-Line Treatment: High-dose Amoxicillin (80–90 mg/kg/day).

• Treatment Failure / Beta-Lactamase Coverage: Amoxicillin-Clavulanate (if prior amoxicillin within 30 days or concurrent purulent conjunctivitis).

Streptococcal Pharyngitis ("Strep Throat")

• Pathogen: Streptococcus pyogenes (Group A Beta-Hemolytic Strep / GAS).

• Presentation: Sudden sore throat, fever, tonsillar exudates, anterior cervical lymphadenopathy (absence of cough).

• 1st-Line Treatment: Penicillin V (oral) or Amoxicillin.

• Penicillin Allergy: Azithromycin (macrolide) or Cephalexin (if non-anaphylactic).

• Complications of Non-Treatment: Acute Rheumatic Fever, Post-Streptococcal Glomerulonephritis.

Acute Bacterial Rhinosinusitis (ABRS)

• Pathogens: S. pneumoniae, H. influenzae, M. catarrhalis.

• 1st-Line Treatment: Amoxicillin-Clavulanate (Augustin pattern preferred over plain amoxicillin to cover beta-lactamase producers).

Lower Respiratory Tract Infections (LRTIs)

Community-Acquired Pneumonia (CAP)

Tuberculosis (TB)

Community-Acquired Pneumonia (CAP)

• Typical Pathogens: Streptococcus pneumoniae (most common), Haemophilus influenzae.

• Atypical Pathogens: Mycoplasma pneumoniae, Legionella pneumophila, Chlamydophila pneumoniae.

• Outpatient Treatment (Healthy, No Comorbidities): Amoxicillin high-dose, Doxycycline, or a Macrolide (Azithromycin/Clarithromycin, only if local pneumococcal resistance <25%).

Community-Acquired Pneumonia (CAP): Outpatient Treatment (With Comorbidities - COPD, Diabetes, Renal/Heart Failure):

• Beta-lactam (Amoxicillin-Clavulanate or Cefpodoxime) + Macrolide or Doxycycline, OR

• Respiratory Fluoroquinolone monotherapy (Levofloxacin 750 mg or Moxifloxacin 400 mg).

Tuberculosis (TB)

• Pathogen: Mycobacterium tuberculosis (Acid-Fast Bacillus).

• Standard Regimen (RIPE Therapy): * Intensive Phase (2 Months): Rifampin + Isoniazid + Pyrazinamide + Ethambutol.

  • Continuation Phase (4 Months): Rifampin + Isoniazid.

Rifampin

Primary Toxicity: Hepatotoxicity, Orange/red secretions

High-Yield Board Pearl: Potent CYP450 inducer; reduces oral contraceptive & warfarin efficacy.

Isoniazid (INH)

Primary Toxicity: Peripheral neuropathy, Hepatotoxicity

High-Yield Board Pearl: Inhibits mycolic acid synthesis. Must co-administer Vitamin B6 (Pyridoxine) to prevent neuropathy.

Pyrazinamide

Primary Toxicity: Hyperuricemia, Hepatotoxicity

High-Yield Board Pearl: Converts to pyrazinoic acid; causes gout flare-ups.

Ethambutol

Primary Toxicity: Optic Neuritis

High-Yield Board Pearl: Inhibits arabinosyl transferase. Causes red-green color blindness and decreased visual acuity.

Urinary Tract Infections (UTIs)

Uncomplicated Cystitis (Lower UTI)

Acute Pyelonephritis (Upper UTI)

Uncomplicated Cystitis (Lower UTI)

• Pathogens: Escherichia coli (~80%), Staphylococcus saprophyticus, Klebsiella pneumoniae, Proteus mirabilis (produces urease → alkaline urine → struvite/staghorn stones).

• Presentation: Dysuria, urinary frequency, urgency, suprapubic pain (no systemic symptoms).

Uncomplicated Cystitis (Lower UTI): 1st-Line Treatment Options

• Nitrofurantoin (100 mg BID x 5 days) — Contraindicated if CrCl <30 mL/min.

• Trimethoprim-Sulfamethoxazole (TMP-SMX) (1 DS tablet BID x 3 days) — Avoid if local resistance >20%.

• Fosfomycin (3 g single oral dose).

Acute Pyelonephritis (Upper UTI)

• Presentation: Flank pain, costovertebral angle (CVA) tenderness, fever, chills, nausea/vomiting.

• 1st-Line Outpatient Treatment: Ciprofloxacin or Levofloxacin for 5–7 days (or IV Ceftriaxone if inpatient).

Skin and Soft Tissue Infections (SSTIs)

Impetigo

Cellulitis vs. Erysipelas

Abscesses & Purulent Wound Infections

Impetigo

• Pathogens: Staphylococcus aureus, Streptococcus pyogenes.

• Presentation: "Honey-crusted" lesions around the mouth and nose in pediatric patients.

• Treatment: Topical Mupirocin ointment for localized disease; oral Cephalexin for widespread lesions.

Cellulitis vs. Erysipelas

• Cellulitis: Deep dermis/subcutaneous tissue infection; poorly demarcated, flat erythematous borders (S. aureus, S. pyogenes). Treatment: Cephalexin or Dicloxacillin.

• Erysipelas: Superficial dermis infection; sharply demarcated, raised, bright red borders (S. pyogenes). Treatment: Penicillin VK or Cephalexin.

Abscesses & Purulent Wound Infections

• Primary Pathogen: Staphylococcus aureus (including Methicillin-Resistant S. aureus / MRSA).

• Primary Intervention: Incision and Drainage (I&D) is the definitive treatment.

• Empiric Oral Coverage for MRSA: TMP-SMX, Doxycycline, or Clindamycin.

• Empiric IV Coverage for Severe MRSA: Vancomycin, Daptomycin, or Linezolid

Gastrointestinal Infections

Clostridioides difficile Infection (CDI)

Infectious Gastroenteritis

Clostridioides difficile Infection (CDI)

• Mechanism: Toxin A (enterotoxin) and Toxin B (cytotoxin) production causing pseudomembranous colitis, often triggered by broad-spectrum antibiotic use (Clindamycin, Fluoroquinolones, Cephalosporins).

• Presentation: Profuse, watery diarrhea ($>3$ loose stools in 24 hours), abdominal cramping, leukocytosis.

• 1st-Line Treatment: Oral Vancomycin (125 mg QID x 10 days) or Oral Fidaxomicin (200 mg BID x 10 days).

• _{^{Exam Pearl: IV Vancomycin is completely ineffective for CDI because it is not excreted into the intestinal lumen.}}

Infectious Gastroenteritis

• Campylobacter jej*ni: Associated with raw poultry/unpasteurized milk; presents with blo*dy diarrhea. Treatment: Azithromycin.

• Salmonella / Shigella: Inflammatory diarrhea. Treatment: Ceftriaxone or Fluoroquinolones.

• Traveler's Diarrhea (ETEC - Enterotoxigenic E. coli): Watery diarrhea. Treatment: Rifaximin or Azithromycin.

S*xually Transmitted Infections (STIs)

Syphilis

Gonorrhea

Chlamydia

Trichomoniasis

Syphilis

• Pathogen: Treponema pallidum (Spirochete).

• Stages: Primary (painless chancre), Secondary (disseminated maculopapular rash including palms/soles, condyloma lata), Tertiary (gummas, neurosyphilis).

• 1st-Line Treatment: Benzathine Penicillin G (2.4 million units IM single dose for primary/secondary/early latent).

• Neurosyphilis Treatment: Aqueous Crystalline Penicillin G IV for 10–14 days

Gonorrhea

• Pathogen: Neisseria gonorrhoeae (Gram-negative diplococci).

• Presentation: Purulent urethral/cervical discharge, dysuria.

• 1st-Line Treatment: Ceftriaxone (500 mg IM single dose for patients $<150\text{ kg}$).

• Co-infection Pearl: If Chlamydia has not been ruled out, add Doxycycline (100 mg BID x 7 days).

Chlamydia

• Pathogen: Chlamydia trachomatis (Obligate intracellular bacterium).

• Presentation: Often asymptomatic; clear/mucopurulent discharge, dysuria, pelvic inflammatory disease (PID).

• 1st-Line Treatment: Doxycycline (100 mg BID x 7 days). Azithromycin (1 g single dose) is reserved as an alternative, preferred in pregnancy.

Trichomoniasis

• Pathogen: Trichomonas vaginalis (Flagellated protozoan).

• Presentation: Frothy yellow-green, foul-smelling vaginal discharge with a "strawberry cervix."

• 1st-Line Treatment: Metronidazole (500 mg BID x 7 days or 2 g single oral dose). Both sexual partners must be treated simultaneously.