LEC Micro Case Study

A patient has low blood pressure and a high fever. What IV fluid is most appropriate, and why?

Isotonic IV fluid. It increases fluid volume in the bloodstream without causing major movement of water into or out of cells. It is used to help restore blood pressure and circulating volume during dehydration or fluid loss.

A patient has a brain injury and high sodium levels. Why might a hypertonic IV fluid be used?

A hypertonic solution has a higher solute concentration than the surrounding cells. It draws water out of cells and into the bloodstream, which can help reduce cellular swelling. In the case of brain injury, this can help decrease brain swelling/edema.

A patient has a UTI and is told to drink fluids and possibly use cranberry products. How can these help?

Increased fluids help increase urination and flush bacteria from the urinary tract. Cranberry products may help by reducing bacterial attachment to urinary tract cells, making it harder for some bacteria to establish infection.

A patient has severe vomiting and diarrhea followed by dehydration. What is happening physiologically?

The patient loses water and electrolytes, disrupting normal fluid balance. Osmosis causes water to shift between body compartments depending on solute concentrations. Severe fluid/electrolyte loss can cause dehydration, weakness, low blood pressure, and electrolyte abnormalities.

A postoperative patient has poor appetite and slow wound healing. What nutritional deficiency should you consider?

Protein deficiency. Protein is needed to build and repair tissues, produce enzymes and immune components, and support wound healing. Inadequate protein can therefore lead to delayed wound healing.

A patient develops rapidly worsening pain, swelling, and tissue destruction after a puncture wound at a nail salon. What condition should you suspect?

Necrotizing fasciitis. This is a rapidly progressing infection that destroys fascia and surrounding soft tissue. Severe pain and rapidly worsening tissue damage are major warning signs. It is a medical emergency requiring rapid treatment, often including surgical removal of infected tissue.

A patient has fever and a heart murmur after dental surgery and has a history of rheumatic fever. What infection should you suspect?

Subacute endocarditis. This is an infection involving the inner lining of the heart and/or heart valves. A damaged heart valve increases susceptibility. Important clues include fever + heart murmur + a cardiac risk factor.

A patient has an infection, severe hypotension, and widespread inflammation. What condition should you suspect?

Sepsis/septic shock. A severe infection can trigger a widespread inflammatory response. Blood vessels dilate and become more permeable, causing blood pressure to fall and tissue perfusion to decrease. Severe cases can progress to organ failure and septic shock.

A patient develops fever, dry cough, and body aches after traveling to Hong Kong and visiting an animal market. What should you suspect?

Influenza A, with concern for a new strain produced through antigenic shift. Influenza A can infect multiple animal species, allowing different viral strains to exchange genetic material. A major genetic reassortment can create a new strain to which humans have little immunity.

Key clue: animal exposure + respiratory illness + unusual/new outbreak = think Influenza A and antigenic shift.

An infant develops severe coughing fits, cyanosis, and episodes of apnea. What disease should you suspect?

Pertussis (whooping cough) caused by Bordetella pertussis.

Important clues:

• Infant/young child

• Severe coughing episodes

• Cyanosis

• Apnea

• Characteristic "whooping" cough may occur

Pertussis can be particularly dangerous in infants because of apnea and difficulty breathing.

A child develops bloody diarrhea after eating undercooked ground beef and later develops kidney problems. What organism and complication should you suspect?

E. coli O157:H7/STEC → Hemolytic Uremic Syndrome (HUS).

The organism produces Shiga toxin, which can damage blood vessels and cause:

• Destruction of red blood cells

• Low platelet count

• Kidney damage

🚨 Critical exam point:

Do NOT give antibiotics for suspected STEC O157:H7, because antibiotic treatment can increase toxin release and potentially worsen the risk of HUS.

Clue pattern:
Ground beef → bloody diarrhea → kidney problems = STEC → HUS

A large outbreak of watery diarrhea is associated with contaminated fresh produce. What organism should you suspect?

Cyclospora.

Cyclospora is a parasite associated with foodborne outbreaks involving fresh produce.

Typical clue:
Fresh produce + outbreak + prolonged watery diarrhea → Cyclospora.

A patient develops profuse "rice-water" diarrhea. What disease should you suspect and why?

Cholera, caused by Vibrio cholerae.

Cholera toxin causes intestinal cells to secrete large amounts of water and electrolytes into the intestinal tract.

This produces:

• Massive watery diarrhea

• Severe dehydration

• Electrolyte loss

• Potential shock

Classic clue: rice-water diarrhea = cholera.

A patient develops bloody diarrhea after eating undercooked beef. What type of E. coli should you suspect?

Shiga toxin-producing E. coli (STEC), especially E. coli O157:H7.

It can cause hemorrhagic/bloody diarrhea and may progress to HUS, particularly in children.

A patient has bloody diarrhea, fever, and dysentery. What organism should you consider?

Shigella.

Shigella infects the intestinal tract and can cause:

• Fever

• Abdominal cramps

• Bloody diarrhea

• Dysentery

Clue: bloody diarrhea + fever + dysentery → Shigella.

A patient develops severe diarrhea after taking antibiotics. What organism should you suspect?

Clostridioides difficile (C. difficile).

Antibiotics can destroy normal intestinal microbiota, allowing C. difficile to overgrow.

It produces toxins that damage the colon and can cause antibiotic-associated diarrhea and colitis.

Clue:
Recent antibiotics + diarrhea = C. difficile

A person develops severe vomiting only 4–6 hours after eating contaminated food. What organism is likely responsible?

Staphylococcus aureus food poisoning.

The important clue is the very short incubation period.

S. aureus produces preformed toxins in food, so symptoms can occur rapidly after eating.

Clue:
Food → 4–6 hours → rapid vomiting = S. aureus

A person eats improperly home-canned food and later develops descending/flaccid paralysis. What disease should you suspect?

Botulism, caused by Clostridium botulinum toxin.

The toxin blocks acetylcholine release at neuromuscular junctions, preventing normal muscle contraction.

This causes:

• Weakness

• Flaccid paralysis

• Difficulty swallowing

• Difficulty breathing

• Descending paralysis

Clue:
Home-canned food + descending flaccid paralysis = botulism

A patient has high fever and rose spots. What disease should you suspect?

Typhoid fever, caused by Salmonella Typhi.

A classic clue is high fever associated with rose-colored spots.

A child has severe itching around the anus, especially at night. What parasite should you suspect?

Pinworm (Enterobius vermicularis).

The female worm comes out at night and deposits eggs around the anus, causing intense nighttime perianal itching.

Clue:
Child + nighttime anal itching = pinworm

A patient has cyclic fever, anemia, and a history of mosquito exposure. What disease should you suspect?

Malaria, caused by Plasmodium species.

It is transmitted by the female Anopheles mosquito.

The parasite:

1. Enters the bloodstream.

2. Travels to the liver.

3. Multiplies.

4. Infects red blood cells.

5. Causes RBCs to rupture.

RBC destruction contributes to anemia and recurring/cyclic fever.

Clue:
Anopheles mosquito + cyclic fever + anemia = malaria

What is the complete recognition pattern for STEC/HUS?

Undercooked ground beef → bloody diarrhea → E. coli O157:H7/STEC → Shiga toxin → RBC destruction + kidney injury → HUS.

🚨 Do not give antibiotics for suspected STEC because treatment can increase toxin release and worsen HUS.

What is the complete recognition pattern for necrotizing fasciitis?

Puncture/wound → rapidly increasing severe pain and swelling → rapidly destroying soft tissue/fascia → necrotizing fasciitis.

It is a medical emergency and requires rapid treatment, often including surgical removal of infected tissue.

What is the complete recognition pattern for endocarditis?

Fever + heart murmur + damaged/prosthetic valve or other risk factor → endocarditis.

The infection involves the inner lining of the heart and/or heart valves.

Know:

• Janeway lesions = painless

• Osler nodes = painful

What is the complete recognition pattern for influenza A antigenic shift?

Influenza-like respiratory illness + animal exposure/travel + emergence of an unusual strain → think Influenza A and possible antigenic shift.

Antigenic drift = small gradual mutations.

Antigenic shift = major genetic reassortment → potentially new strain → pandemic potential.

What is the complete recognition pattern for Cyclospora?

Fresh produce exposure + outbreak + watery diarrhea → Cyclospora.

The important association from the case is foodborne outbreaks involving fresh produce.

Low BP + high fever → ?

Isotonic IV fluid.

Brain injury + high sodium → ?

Hypertonic IV fluid.

UTI + cranberry → ?

Fluids + reduced bacterial attachment.

Poor nutrition + slow wound healing → ?

Protein deficiency.

Rapid tissue destruction after wound → ?

Necrotizing fasciitis.

Fever + murmur + cardiac risk → ?

Endocarditis.

Infection + hypotension + systemic inflammation → ?

Sepsis/septic shock.

Animal exposure + influenza symptoms → ?

Influenza A; consider antigenic shift.

Infant + coughing fits + apnea/cyanosis → ?

Pertussis.

Ground beef + bloody diarrhea + kidney injury → ?

STEC O157:H7 → HUS.

Fresh produce + watery diarrhea outbreak → ?

Cyclospora.

Rice-water diarrhea → ?

Cholera.

Bloody diarrhea + fever/dysentery → ?

Shigella.

Antibiotics → diarrhea → ?

C. difficile.

Vomiting 4–6 hours after food → ?

S. aureus.

Home-canned food + descending paralysis → ?

Botulism.

Fever + rose spots → ?

Typhoid.

Nighttime anal itching → ?

Pinworm.

Cyclic fever + mosquito + anemia → ?

Malaria.