Attack and Defense

Histology

100

How will gram (-) vs gram (+) appear under a gram stain?

• Gram-positive bacteria (Gram +)
  • Appearance: Purple / blue
  • Reason:
    • Thick peptidoglycan traps crystal violet–iodine complex
    • Does NOT wash out with alcohol
  • Examples:
    • Staph, Strep, Clostridium
• Gram-negative bacteria (Gram −)
  • Appearance: Pink / red
  • Reason:
    • Thin peptidoglycan loses crystal violet during alcohol wash
    • Then takes up safranin counterstain
  • Examples:
    • E. coli, Pseudomonas, Neisseria

200

A 23-year-old man comes to the physician for a 1-week history of orange discoloration of his urine. Urinalysis shows 1+ blood. He has had no recent illnesses he can recall. Four months ago he went traveling through western Africa for two weeks with a group of friends. Which of the following is the most likely diagnosis and treatment for this patient?

A. Schistosoma haematobium infection — praziquantel
B. Plasmodium falciparum infection — chloroquine
C. Leptospira interrogans infection — doxycycline
D. Trichomonas vaginalis infection — metronidazole

• Key clues
  • Hematuria (orange urine + blood)
  • Travel to Africa
  • Image shows:
    • Egg with terminal spine

• Diagnosis
  • Schistosoma haematobium

• Why this fits
  • Lives in:
    • Venous plexus of bladder
  • Causes:
    • Hematuria
    • Urinary symptoms
  • Egg:
    • Terminal spine (classic)

• Treatment
  • Praziquantel

• Correct answer
  • Schistosoma haematobium infection — praziquantel

• High-yield differentiation
  • S. haematobium
    • Urinary disease
    • Terminal spine
  • S. mansoni
    • Intestinal/liver
    • Lateral spine

• Exam pearl
  • Africa + hematuria + egg with terminal spine → S. haematobium

• One-line memory
  • “HaeMATObium → hematuria → treat with praziquantel”

300

A 24-year-old woman comes to the physician for a 1-week history of blistering skin lesions on her arms, trunk, and legs. A biopsy specimen of the skin shows subepidermal blisters. One year ago she was treated with trimethoprim/sulfamethoxazole for a urinary tract infection. Vitals are normal. Which of the following is the most likely explanation for this patient’s findings?

A. Bullous pemphigoid
B. Goodpasture syndrome
C. Granulomatosis with polyangiitis
D. Pemphigus vulgaris
E. Stevens-Johnson syndrome

A = Bullous pemphigoid

B = Pemphigus vulgaris

400

A 46-year-old man comes to the physician for a 4-day history of fever. He recently immigrated to the USA from Nigeria. He appears lethargic. Temperature is 103°F. Thick and thin blood smears are ordered. A photomicrograph is shown.

Which of the following is the most likely diagnosis?

A. Babesia microti infection
B. Plasmodium falciparum infection
C. Trypanosoma brucei infection
D. Leishmania donovani infection

• Key clues
  • Recent immigration from Nigeria (endemic area)
  • High fever
  • Blood smear shows:
    • Ring forms inside RBCs
    • Multiple rings per cell / delicate forms (typical)

• Diagnosis
  • Plasmodium falciparum infection

• Why this is correct
  • Most common severe malaria in Africa
  • Smear findings:
    • Multiple ring forms per RBC
    • No RBC enlargement
  • Can cause:
    • Severe disease
    • Cerebral malaria

• Why others are wrong
  • Babesia
    • Tick exposure (Northeast US), Maltese cross
  • Trypanosoma brucei
    • Seen in blood as extracellular parasites
  • Leishmania
    • Intracellular in macrophages, not RBCs

• High-yield takeaway
  • Africa + severe malaria + ring forms → Plasmodium falciparum 

500

A 66-year-old man comes to the physician for a 2-day history of high fever and severe leg pain. Physical examination shows darkening of the skin of the leg and crepitus. He has a 30-year history of type II diabetes mellitus and hypertension managed with multiple medications. Vitals are: temperature 103°F, HR 80, RR 20, BP 150/90. A gram stain of the causal organism is shown.

Which of the following is the most likely explanation for this patient’s findings?

A. Coagulase
B. Exotoxin A
C. Phospholipase C
D. Lipid A
E. Lipoteichoic acid

Clostridium perfringens

• Gram-positive anaerobic rod
• α-toxin = phospholipase C (lecithinase)
  • Breaks down cell membrane phospholipids
  • → myonecrosis
  • → hemolysis
  • → gas formation 

• Why others are wrong
  • Coagulase
    • Staph aureus → abscesses, not gas gangrene
  • Exotoxin A
    • Pseudomonas → inhibits EF-2
  • Lipid A
    • Gram-negative endotoxin → septic shock
  • Lipoteichoic acid
    • Gram-positive cell wall component, not toxin causing necrosis