Can't Breathe
A patient with influenza develops worsening fever, cough, and cavitary lung lesions. This pathogen should be suspected.
Staphylococcus aureus (especially MRSA)
Post-influenza bacterial pneumonia is commonly due to S. aureus, which can cause necrotizing pneumonia with cavitation, severe hypoxia, and rapid deterioration. Treat with MRSA-active agents: vancomycin or linezolid plus standard CAP therapy.
Preferred timing of initiation of ART with a new diagnosis of HIV.
Within 2 weeks of initial diagnosis.
All persons with HIV infection should begin ART as soon as they are ready, regardless of CD4 cell count. Rapid initiation of ART has been shown to improve viral suppression and should be considered if no medical (symptoms suggesting opportunistic infections in which immediate ART is contraindicated) or structural (staffing and linkage to care service availability) barriers prevent doing so.
This is the most common pathogen causing non-purulent cellulitis in an otherwise healthy adult.
Streptococcus pyogenes (Group A Streptococcus)
Non-purulent cellulitis is usually due to beta-hemolytic streptococci (mainly GAS). Empiric treatment should target streptococci (e.g., cefazolin, ceftriaxone). If purulent, suspect Staphylococcus aureus (including MRSA).
This is the most common bacterial cause of community-acquired bacterial meningitis in adults.
Streptococcus pneumoniae
S. pneumoniae is the leading cause of bacterial meningitis in adults. Empiric treatment: IV vancomycin + third-generation cephalosporin (e.g., ceftriaxone) ± ampicillin (if >50 years or immunocompromised, to cover Listeria).
A patient from Phoenix, AZ develops cough, fever, and erythema nodosum. CXR shows hilar adenopathy. This is the most likely etiology, and moderate disease should be treated with this.
Coccidioides immitis, fluconazole (or itraconazole).
Coccidioidomycosis or valley fever → desert Southwest, inhaled spores → pneumonia ± rash (erythema nodosum). Mild or asymptomatic: supportive treatment. Moderate/Severe: Fluconazole.
This clinical scoring tool helps determine if a patient with pneumonia can be treated as an outpatient or needs admission.
PSI (pneumonia severity index) or CURB 65 score.
CURB-65: Confusion, Urea >7 mmol/L, Respiratory rate ≥30, BP low (<90/60), age ≥65. Higher scores → higher mortality risk → consider inpatient or ICU care.
42F with HIV on ART. Most recent labs showing undetectable HIV-1 quantitative RNA, same from 6 months and 1 year ago. The last CD4 cell count was 650/µL.
Most appropriate management to prevent HIV transmission.
No additional preventive strategy.
The risk of HIV transmission is essentially zero in patients who have a sustained, undetectable viral load for at least 6 months (Undetectable = Untransmittable), so monogamous, serodifferent partners do not require further HIV prevention measures.
X-Ray, MRI w/ and w/o contrast.
Plain radiography can confirm the diagnosis in most patients if typical findings are present but cannot exclude the diagnosis if negative. Because of its low cost and specificity, plain radiography is recommended as an initial imaging test. If a plain radiograph is not diagnostic, MRI with and without intravenous contrast is preferred. Bone biopsy with culture is the gold standard for diagnosis and guiding targeted therapy.
A patient with new-onset seizures and temporal lobe edema on MRI likely has encephalitis due to this virus.
Herpes simplex virus type 1 (HSV-1)
HSV-1 is the most common cause of sporadic fatal encephalitis in adults. It causes hemorrhagic necrosis of the temporal lobes → seizures, personality changes, focal neurologic deficits. Treat with IV acyclovir immediately — don’t wait for PCR confirmation.
A patient with asthma, eosinophilia and recurrent bronchial plugging with brownish sputum should be evaluated for this condition.
Allergic Bronchopulmonary Aspergillosis (ABPA)
ABPA = asthma or CF + IgE-mediated reaction to Aspergillus. Clues: eosinophilia, fleeting infiltrates, elevated IgE. Steroids are first-line; add itraconazole if severe or recurrent.
A college student develops pneumonia over 2 weeks with dry cough, mild fever, and cold agglutinins on lab work. This pathogen is likely.
Mycoplasma pneumoniae
“Walking pneumonia” — mild symptoms, patchy CXR, positive cold agglutinins. Lacks a cell wall → beta-lactams don’t work. Use macrolides or doxycycline.
A patient with HIV and CD4 count <200 should receive this prophylactic medication to prevent Pneumocystis jirovecii pneumonia (PJP).
TMP-SMX
Patients with CD4 <200 cells/µL are at high risk for PJP. TMP-SMX is first-line for prophylaxis and can also prevent Toxoplasma gondii when CD4 <100 (if IgG positive). If intolerant, alternatives include dapsone, atovaquone, or aerosolized pentamidine.
A patient develops rapid-onset pain, swelling, and crepitus at a wound site. This anaerobic pathogen is the classic cause of gas gangrene.
Clostridium perfringens.
Gas gangrene (clostridial myonecrosis) is a surgical emergency characterized by rapidly progressive muscle necrosis, gas production, and sepsis. Treatment includes immediate surgical debridement, high-dose IV penicillin plus clindamycin (to inhibit toxin production), and supportive care.
A 23-year-old college student presents with fever, headache, petechial rash, and nuchal rigidity. This bacterial pathogen is most likely.
Neisseria meningitidis
Neisseria meningitidis causes outbreaks in young adults, especially in close quarters (dorms, barracks). Look for petechial or purpuric rash — a classic clue for meningococcemia. Close contacts need rifampin, ciprofloxacin, or ceftriaxone prophylaxis.
A diabetic patient with DKA develops sinus pain, black nasal eschar, and orbital swelling. This diagnosis should receive this urgent management.
Mucormycosis, emergent surgical debridement plus IV amphotericin B.
Mucormycosis (Rhizopus) loves DKA, immunosuppression → rapidly spreads through sinuses/orbit → can reach brain. Surgery + IV amphotericin B is life-saving. Control glucose/acidosis urgently.
For a patient with healthcare-associated or hospital-acquired pneumonia at risk for multidrug-resistant organisms, the initial antibiotic regimen should include these two broad principles.
MRSA coverage and double Pseudomonas coverage ?
HAP/VAP often requires empiric MRSA coverage and anti-pseudomonal coverage.
Two antipseudomonal agents of different classes are recommended for empiric regimens only for patients with risk factors for resistance, with structural lung disease (bronchiectasis, cystic fibrosis), or in a unit with greater than 10% resistance to an agent being considered for monotherapy.
A 32-year-old man with HIV presents with fever, headache, and neck stiffness. CSF shows lymphocytic pleocytosis and elevated opening pressure.
This test confirms the most likely diagnosis.
Cryptococcal antigen test (CrAg) in CSF and serum.
Cryptococcus neoformans is an encapsulated yeast causing meningitis, especially in patients with advanced HIV (CD4 <100). India ink can reveal the capsule in up to 50% of cases, but a cryptococcal antigen test (CrAg) in CSF and serum is more sensitive and is the preferred confirmatory test today.
A fisherman with PMH cirrhosis has a hand wound with hemorrhagic bullae and severe sepsis after exposure to warm seawater. Suspected pathogen and treatment.
Vibrio vulnificus. Doxycycline + third-gen cephalosporin.
Vibrio vulnificus is a Gram-negative bacterium found in warm coastal waters. Wound infections can rapidly progress to necrotizing fasciitis and sepsis, especially in patients with liver disease. Treatment is doxycycline plus a third-generation cephalosporin.
In suspected bacterial meningitis, this medication should be given before or with the first dose of empiric antibiotics to reduce mortality and neurologic complications
Dexamethasone
Adjunctive dexamethasone reduces inflammation, decreasing hearing loss and poor outcomes in S. pneumoniae meningitis. Give before or with the first antibiotic dose; stop if S. pneumoniae is ruled out.
A patient with a history of old TB has hemoptysis and a mobile mass in a lung cavity. This is the most likely cause and management may require this.
Aspergilloma, managed with surgical resection if bleeding is severe.
Aspergilloma = Aspergillus grows in pre-existing cavities → fungus ball → hemoptysis risk. Surgery for massive bleed; antifungals don’t reliably cure it.
A patient with pneumonia develops persistent fever and worsening dyspnea despite appropriate antibiotics. Chest X-ray shows a large pleural effusion. Thoracentesis reveals purulent fluid with pH <7.2 and glucose <60 mg/dL. This complication should be suspected and requires this key management step.
empyema, chest tube drainage (or surgical drainage if needed)
Empyema is a pus-filled pleural space, a complication of pneumonia. It must be drained because antibiotics alone won’t sterilize a closed infected cavity. Exudative criteria: low pH (<7.2), low glucose (<60), high LDH, positive Gram stain or culture. If thick or loculated, consider intrapleural fibrinolytics or VATS decortication.
An HIV-positive patient with chronic watery diarrhea and CD4 count of 40 is most likely infected with this protozoan parasite, which can be diagnosed by acid-fast staining of stool.
Cryptosporidium parvum.
Cryptosporidium causes severe, chronic watery diarrhea in advanced HIV (CD4 <100). It’s detected by acid-fast stain (oocysts) or stool antigen tests. There’s no fully effective treatment, so initiation and maintenance of ART is key.
A patient with PMH splenectomy suffers a dog bite and develops signs of severe infection with foul-smelling discharge and crepitus. This anaerobic organism can be part of dog oral flora and cause severe wound infections.
Capnocytophaga canimorsus
Capnocytophaga canimorsus is a Gram-negative rod found in dog and cat saliva. It can cause severe wound infections, bacteremia, or sepsis — especially in asplenic, alcoholic, or immunocompromised patients. Bite wounds are usually polymicrobial: cover for Pasteurella, anaerobes, and Capnocytophaga with amoxicillin-clavulanate as first-line therapy.
An HIV patient with CD4 <100 develops headache, fever, and elevated CSF opening pressure. India ink stain shows encapsulated yeast. This antifungal induction therapy is required.
Amphotericin B + flucytosine
Cryptococcus neoformans causes cryptococcal meningitis in advanced HIV. Induction: IV amphotericin B + oral flucytosine for 2 weeks → consolidation with fluconazole for at least 8 weeks → maintenance until CD4 recovery. Always manage raised intracranial pressure with repeated LPs — it’s a major cause of death.
A gardener develops a painless ulcer on the hand that progresses to ascending nodular lesions. This infection should be treated with this first-line medication.
Sporotrichosis, itraconazole.
Sporothrix schenckii → “rose gardener’s disease.” Subcutaneous nodules spread along lymphatics (lymphocutaneous). Diagnose by culture from pus or biopsy. First-line: itraconazole 3–6 months. For severe disseminated cases (rare, usually immunocompromised): amphotericin B.