A 3-year-old female is found to have unusual susceptibility to infections by catalase-producing organisms. This patient likely has a problem with the function of which of the following cell types?
Q62
A 28-year-old woman comes to the physician because of a 4-day history of lower abdominal pain and pain with urination. Five months ago, she was treated for gonococcal urethritis. She recently moved in with her newlywed husband. She is sexually active with her husband and they do not use condoms. Her only medication is an oral contraceptive. Her temperature is 37.5°C (99.7°F) and blood pressure is 120/74 mm Hg. There is tenderness to palpation over the pelvic region. Pelvic examination shows a normal-appearing vulva and vagina. Laboratory studies show:
Leukocyte count 8,400/mm3
Urine
pH 6.7
Protein trace
WBC 60/hpf
Nitrites positive
Bacteria positive
Which of the following is the most likely causal organism?
Q63
A 12-year-old boy admitted to the intensive care unit 1 day ago for severe pneumonia suddenly develops hypotension. He was started on empiric antibiotics and his blood culture reports are pending. According to the nurse, the patient was doing fine until his blood pressure suddenly dropped. Vital signs include: blood pressure is 88/58 mm Hg, temperature is 39.4°C (103.0°F), pulse is 120/min, and respiratory rate is 24/min. His limbs feel warm. The resident physician decides to start him on intravenous vasopressors, as the blood pressure is not responding to intravenous fluids. The on-call intensivist suspects shock due to a bacterial toxin. What is the primary mechanism responsible for the pathogenesis of this patient's condition?
Q64
A 32-year-old man comes to the emergency department because of nausea and vomiting for the past 2 hours. The patient has neither had diarrhea nor fever. Four hours ago he ate some leftover Indian rice dish he had ordered the night before. There is no history of serious illness. He immigrated from India 8 years ago with his family and now works as a butcher. He appears ill. His temperature is 36.7°C (98°F), pulse is 85/min, and blood pressure is 115/70 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 98%. Which of the following is the most likely causal organism?
Q65
A researcher is studying the effects of various substances on mature B-cells. She observes that while most substances are only able to promote the production of antibodies when the B-cells are co-cultured with T-cells, a small subset of substances are able to trigger antibody production even in the absence of T-cells. She decides to test these substances that stimulate B-cells alone by injecting them into model organisms. She then analyzes the characteristics of the response that is triggered by these substances. Which of the following correctly describes how the immune response triggered by the B-cell-alone-substances compares with that triggered by substances that also require T-cells?
Q66
A 50-year-old male presents to the emergency room complaining of fever, shortness of breath, and diarrhea. He returned from a spa in the Rocky Mountains five days prior. He reports that over the past two days, he developed a fever, cough, dyspnea, and multiple watery stools. His past medical history is notable for major depressive disorder and peptic ulcer disease. He takes omeprazole and paroxetine. He does not smoke and drinks alcohol on social occasions. His temperature is 102.8°F (39.3°C), blood pressure is 120/70 mmHg, pulse is 65/min, and respirations are 20/min. Physical examination reveals dry mucus membranes, delayed capillary refill, and rales at the bilateral lung bases. A basic metabolic panel is shown below:
Serum:
Na+: 126 mEq/L
Cl-: 100 mEq/L
K+: 4.1 mEq/L
HCO3-: 23 mEq/L
Ca2+: 10.1 mg/dL
Mg2+: 2.0 mEq/L
Urea nitrogen: 14 mg/dL
Glucose: 90 mg/dL
Creatinine: 1.1 mg/dL
Which of the following is the most appropriate growth medium to culture the pathogen responsible for this patient’s condition?
Q67
A 26-year-old woman presents to the emergency department with confusion, severe myalgia, fever, and a rash over her inner left thigh. The patient was diagnosed with pharyngitis three days ago and prescribed antibiotics, but she did not take them. Her blood pressure is 90/60 mm Hg, heart rate is 99/min, respiratory rate is 17/min, and temperature is 38.9°C (102.0°F). On physical examination, the patient is disoriented. The posterior wall of her pharynx is erythematous and swollen and protrudes into the pharyngeal lumen. There is a diffuse maculopapular rash over her thighs and abdomen. Which of these surface structures interacts with the causative agent of her condition?
Q68
A 32-year-old man with hypertension and gout comes to the physician with left flank pain and bloody urine for two days. He does not smoke cigarettes but drinks two beers daily. Home medications include hydrochlorothiazide and ibuprofen as needed for pain. Physical examination shows left costovertebral angle tenderness. Urine dipstick is strongly positive for blood. Microscopic analysis of a stone found in the urine reveals a composition of magnesium ammonium phosphate. Which of the following is the strongest predisposing factor for this patient's condition?
Q69
A 72-year-old patient presents to the emergency department because of abdominal pain, diarrhea, and fever. He was started on levofloxacin for community-acquired pneumonia 2 weeks prior with resolution of his pulmonary symptoms. He has had hypertension for 20 years, for which he takes amlodipine. His temperature is 38.3°C (101.0°F), pulse is 90/min, and blood pressure is 110/70 mm Hg. On examination, mild abdominal distension with minimal tenderness was found. Laboratory tests reveal a peripheral white blood cell count of 12.000/mm3 and a stool guaiac mildly positive for occult blood. Which of the following best describe the mechanism of this patient illness?
Q70
A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1°F (38.9°C), pulse is 56/min, respirations are 16/min, and blood pressure is 150/85 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show:
Hemoglobin 13.5 g/dL
Leukocyte count 15,000/mm3
Platelet count 130,000/mm3
Serum
Na+ 129 mEq/L
Cl- 100 mEq/L
K+ 4.6 mEq/L
HCO3- 22 mEq/L
Urea nitrogen 14 mg/dL
Creatinine 1.3 mg/dL
An x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?
Bacteria US Medical PG Practice Questions and MCQs
Question 61: A 3-year-old female is found to have unusual susceptibility to infections by catalase-producing organisms. This patient likely has a problem with the function of which of the following cell types?
A. Natural killer cells
B. Eosinophils
C. B cells
D. T cells
E. Neutrophils (Correct Answer)
Explanation: ***Neutrophils***
- Catalase-producing organisms, such as *Staphylococcus aureus* or *Aspergillus*, are typically cleared by **phagocytic cells**, specifically **neutrophils**, which use the **respiratory burst** to produce reactive oxygen species.
- A defect in neutrophil function, particularly in the **NADPH oxidase enzyme complex** responsible for the respiratory burst, leads to **chronic granulomatous disease (CGD)**, characterized by increased susceptibility to infections by these specific pathogens.
*Natural killer cells*
- **Natural killer (NK) cells** are primarily involved in the anti-viral and anti-tumor immune responses, recognizing and **killing infected or malignant cells**.
- They do not play a primary role in clearing bacterial or fungal infections, especially those caused by catalase-producing organisms.
*Eosinophils*
- **Eosinophils** are primarily involved in defense against **parasitic infections** and in mediating **allergic reactions**.
- Their role in clearing common bacterial or fungal infections is limited.
*B cells*
- **B cells** are responsible for **humoral immunity**, producing **antibodies** that neutralize pathogens and toxins or opsonize them for phagocytosis.
- While antibodies can aid in the clearance of many pathogens, a primary deficiency in B cell function (e.g., agammaglobulinemia) would lead to broad susceptibility to encapsulated bacteria, not specifically catalase-positive organisms.
*T cells*
- **T cells** are central to **cell-mediated immunity**, recognizing and eliminating intracellular pathogens or directly killing infected cells.
- Deficiencies in T cell function (e.g., SCID) lead to severe immunodeficiency with susceptibility to opportunistic infections, but do not specifically point to problems with catalase-producing organisms as the hallmark.
Question 62: A 28-year-old woman comes to the physician because of a 4-day history of lower abdominal pain and pain with urination. Five months ago, she was treated for gonococcal urethritis. She recently moved in with her newlywed husband. She is sexually active with her husband and they do not use condoms. Her only medication is an oral contraceptive. Her temperature is 37.5°C (99.7°F) and blood pressure is 120/74 mm Hg. There is tenderness to palpation over the pelvic region. Pelvic examination shows a normal-appearing vulva and vagina. Laboratory studies show:
Leukocyte count 8,400/mm3
Urine
pH 6.7
Protein trace
WBC 60/hpf
Nitrites positive
Bacteria positive
Which of the following is the most likely causal organism?
A. Enterococcus faecalis
B. Klebsiella pneumoniae
C. Neisseria gonorrhoeae
D. Escherichia coli (Correct Answer)
E. Staphylococcus saprophyticus
Explanation: ***Escherichia coli***
- The presence of **nitrites** in the urine analysis strongly suggests a urinary tract infection caused by a **nitrate-reducing bacterium**, such as *E. coli*.
- *E. coli* is the **most common cause of uncomplicated UTIs**, especially in sexually active women, and the symptoms (lower abdominal pain, dysuria, pyuria, bacteriuria) are classic for a UTI.
*Enterococcus faecalis*
- While *Enterococcus faecalis* can cause UTIs, it is **less common** than *E. coli* in uncomplicated cases and typically **does not produce nitrites** in urine due to lacking nitrate reductase.
- It is more commonly associated with UTIs in hospitalized patients or those with urinary tract abnormalities.
*Klebsiella pneumoniae*
- *Klebsiella pneumoniae* can cause UTIs and is a **nitrite-producing bacterium**, but it is a **less frequent cause** of community-acquired uncomplicated UTIs compared to *E. coli*.
- It is more often associated with healthcare-associated infections or UTIs in compromised hosts.
*Neisseria gonorrhoeae*
- While the patient has a history of gonococcal urethritis, current symptoms are more consistent with a UTI, and *Neisseria gonorrhoeae* is a **rare cause of cystitis** or pyelonephritis.
- Gonorrhea primarily causes urethritis, cervicitis, or pelvic inflammatory disease, and **does not typically produce nitrites** from nitrates in urine.
*Staphylococcus saprophyticus*
- *Staphylococcus saprophyticus* is a common cause of UTIs in young, sexually active women, but it is **nitrite-negative** because it does not possess nitrate reductase.
- The positive nitrites in the urine make *E. coli* a more likely culprit in this case.
Question 63: A 12-year-old boy admitted to the intensive care unit 1 day ago for severe pneumonia suddenly develops hypotension. He was started on empiric antibiotics and his blood culture reports are pending. According to the nurse, the patient was doing fine until his blood pressure suddenly dropped. Vital signs include: blood pressure is 88/58 mm Hg, temperature is 39.4°C (103.0°F), pulse is 120/min, and respiratory rate is 24/min. His limbs feel warm. The resident physician decides to start him on intravenous vasopressors, as the blood pressure is not responding to intravenous fluids. The on-call intensivist suspects shock due to a bacterial toxin. What is the primary mechanism responsible for the pathogenesis of this patient's condition?
A. Inactivation of elongation factor (EF) 2
B. Inhibition of GABA and glycine
C. Inhibition of acetylcholine release
D. Release of tumor necrosis factor (TNF) (Correct Answer)
E. Degradation of lecithin in cell membranes
Explanation: ***Release of tumor necrosis factor (TNF)***
- The patient's presentation with **warm limbs** and **hypotension** despite fluid resuscitation in the setting of severe pneumonia is highly suggestive of **septic shock**.
- **Bacterial toxins**, particularly **endotoxins** from gram-negative bacteria or **exotoxins** like superantigens, trigger a massive **inflammatory response** by stimulating immune cells to release pro-inflammatory cytokines such as **TNF-α**, IL-1, and IL-6, leading to systemic vasodilation and capillary leak.
*Inactivation of elongation factor (EF) 2*
- This is the mechanism of action of **diphtheria toxin** and **exotoxin A** from *Pseudomonas aeruginosa*.
- While these toxins can cause severe systemic illness, their primary role is not typically the induction of septic shock characterized by widespread vasodilation and warm extremities.
*Inhibition of GABA and glycine*
- This mechanism is characteristic of **tetanus toxin**, which prevents the release of inhibitory neurotransmitters and leads to spastic paralysis.
- This is not consistent with the patient's presentation of septic shock.
*Inhibition of acetylcholine release*
- This is the mechanism of action of **botulinum toxin**, which causes flaccid paralysis by blocking acetylcholine release at the neuromuscular junction.
- This effect is not associated with the pathogenesis of septic shock.
*Degradation of lecithin in cell membranes*
- This mechanism is associated with **alpha toxin** of *Clostridium perfringens* (lecithinase), which causes gas gangrene and hemolysis.
- While this toxin contributes to tissue damage in certain infections, it is not the primary mechanism behind the systemic inflammatory response and vasodilation seen in septic shock.
Question 64: A 32-year-old man comes to the emergency department because of nausea and vomiting for the past 2 hours. The patient has neither had diarrhea nor fever. Four hours ago he ate some leftover Indian rice dish he had ordered the night before. There is no history of serious illness. He immigrated from India 8 years ago with his family and now works as a butcher. He appears ill. His temperature is 36.7°C (98°F), pulse is 85/min, and blood pressure is 115/70 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 98%. Which of the following is the most likely causal organism?
A. Shigella dysenteriae
B. Bacillus cereus (Correct Answer)
C. Staphylococcus aureus
D. Vibrio parahaemolyticus
E. Enterohemorrhagic Escherichia coli
Explanation: ***Bacillus cereus***
- The patient's symptoms of rapid-onset nausea and vomiting (1-5 hours after ingestion) without diarrhea or fever, following consumption of a **rice dish**, are highly characteristic of the emetic toxin produced by *Bacillus cereus*.
- This **cereulide toxin** is heat-stable and pre-formed in food, leading to quick onset, often associated with improperly stored or reheated rice.
*Shigella dysenteriae*
- This organism typically causes **bloody diarrhea**, fever, and severe abdominal cramps (bacillary dysentery), not primarily nausea and vomiting without diarrhea.
- The incubation period is usually longer (24-72 hours) than the rapid onset seen in this patient.
*Staphylococcus aureus*
- While *S. aureus* can cause rapid-onset nausea and vomiting due to pre-formed **enterotoxins**, it is more commonly associated with protein-rich foods like deli meats, dairy products, and custards, rather than rice dishes.
- The symptoms can be very similar to *Bacillus cereus* emetic type, but the food source points away from *S. aureus* in this specific scenario.
*Vibrio parahaemolyticus*
- This pathogen is associated with consumption of raw or undercooked **seafood** and typically causes watery diarrhea, abdominal cramps, nausea, vomiting, and fever.
- The patient ate a rice dish, not seafood, and his primary symptoms are nausea and vomiting without diarrhea.
*Enterohemorrhagic Escherichia coli*
- EHEC typically causes severe **abdominal cramps**, **bloody diarrhea** (hemorrhagic colitis), and may lead to hemolytic uremic syndrome (HUS).
- Nausea and vomiting can occur but are usually accompanied by bloody diarrhea, which is absent in this patient.
Question 65: A researcher is studying the effects of various substances on mature B-cells. She observes that while most substances are only able to promote the production of antibodies when the B-cells are co-cultured with T-cells, a small subset of substances are able to trigger antibody production even in the absence of T-cells. She decides to test these substances that stimulate B-cells alone by injecting them into model organisms. She then analyzes the characteristics of the response that is triggered by these substances. Which of the following correctly describes how the immune response triggered by the B-cell-alone-substances compares with that triggered by substances that also require T-cells?
A. Is T-dependent
B. Is T-independent (Correct Answer)
C. Requires MHC class II presentation
D. Requires cognate interaction
E. Requires B7-CD28 interaction
Explanation: ***Is T-independent***
- The scenario describes substances that activate B-cells and induce antibody production **without the need for T-cells**, which is the defining characteristic of a T-independent immune response.
- T-independent activation typically involves **polysaccharide antigens** or other repetitive structures that can cross-link multiple B-cell receptors, providing a strong enough signal for activation without T-cell help.
*Is T-dependent*
- This option is incorrect because the question explicitly states that the substances can trigger antibody production **in the absence of T-cells**.
- A T-dependent response requires **CD4+ T-helper cells** to activate B-cells through co-stimulation and cytokine signaling.
*Requires MHC class II presentation*
- While B-cells can present antigens via **MHC class II** to T-cells in T-dependent responses, T-independent activation of B-cells does not necessarily require antigen presentation on MHC class II molecules.
- T-independent antigens often directly activate B-cells through **toll-like receptors (TLRs)** or extensive cross-linking of B-cell receptors.
*Requires cognate interaction*
- **Cognate interaction** refers to the specific recognition between an antigen-presenting B-cell and a helper T-cell, which is a hallmark of T-dependent responses.
- Since the B-cells are producing antibodies in the absence of T-cells, cognate interaction is not required in this specific scenario.
*Requires B7-CD28 interaction*
- The **B7-CD28 interaction** is a crucial co-stimulatory signal provided by antigen-presenting cells (like B-cells) to T-cells, and from T-cells back to B-cells, in a T-dependent immune response.
- As the scenario involves T-cell independent activation, this co-stimulatory pathway between T-cells and B-cells is not essential for antibody production.
Question 66: A 50-year-old male presents to the emergency room complaining of fever, shortness of breath, and diarrhea. He returned from a spa in the Rocky Mountains five days prior. He reports that over the past two days, he developed a fever, cough, dyspnea, and multiple watery stools. His past medical history is notable for major depressive disorder and peptic ulcer disease. He takes omeprazole and paroxetine. He does not smoke and drinks alcohol on social occasions. His temperature is 102.8°F (39.3°C), blood pressure is 120/70 mmHg, pulse is 65/min, and respirations are 20/min. Physical examination reveals dry mucus membranes, delayed capillary refill, and rales at the bilateral lung bases. A basic metabolic panel is shown below:
Serum:
Na+: 126 mEq/L
Cl-: 100 mEq/L
K+: 4.1 mEq/L
HCO3-: 23 mEq/L
Ca2+: 10.1 mg/dL
Mg2+: 2.0 mEq/L
Urea nitrogen: 14 mg/dL
Glucose: 90 mg/dL
Creatinine: 1.1 mg/dL
Which of the following is the most appropriate growth medium to culture the pathogen responsible for this patient’s condition?
A. Eaton’s agar
B. Bordet-Gengou agar
C. Thayer-Martin agar
D. Charcoal yeast agar with iron and cysteine (Correct Answer)
E. Sorbitol-MacConkey agar
Explanation: ***Charcoal yeast agar with iron and cysteine***
- This patient's symptoms (fever, cough, dyspnea, diarrhea) after visiting a spa, combined with **hyponatremia** (Na+ 126 mEq/L), are highly characteristic of **Legionnaires' disease** caused by *Legionella pneumophila*.
- *Legionella* is a fastidious organism that requires specialized media for growth, specifically charcoal yeast extract (BCYE) agar supplemented with **L-cysteine** and **iron salts**.
*Eaton’s agar*
- Eaton's agar is a specialized medium used for the primary isolation of **Mycoplasma pneumoniae**.
- *Mycoplasma pneumoniae* typically causes **atypical pneumonia** but does not present with severe gastrointestinal symptoms or hyponatremia as seen in this patient.
*Bordet-Gengou agar*
- Bordet-Gengou agar, containing potato extract, glycerol, and blood, is the selective medium used for the isolation of **Bordetella pertussis**, the causative agent of whooping cough.
- The clinical presentation of **pertussis** involves paroxysmal coughing fits, often with a 'whoop,' and is distinct from the patient's symptoms.
*Thayer-Martin agar*
- Thayer-Martin agar is a selective medium primarily used for the isolation of **Neisseria gonorrhoeae** and *Neisseria meningitidis*.
- These bacteria cause gonorrhea and meningitis, respectively, and are not associated with the respiratory and gastrointestinal symptoms described.
*Sorbitol-MacConkey agar*
- Sorbitol-MacConkey agar is a differential and selective medium used to detect **enterohemorrhagic Escherichia coli O157:H7** in stool samples, which appears as colorless colonies because it cannot ferment sorbitol.
- While the patient has diarrhea, the predominant respiratory symptoms, hyponatremia, and exposure history point away from E. coli O157:H7 as the primary pathogen.
Question 67: A 26-year-old woman presents to the emergency department with confusion, severe myalgia, fever, and a rash over her inner left thigh. The patient was diagnosed with pharyngitis three days ago and prescribed antibiotics, but she did not take them. Her blood pressure is 90/60 mm Hg, heart rate is 99/min, respiratory rate is 17/min, and temperature is 38.9°C (102.0°F). On physical examination, the patient is disoriented. The posterior wall of her pharynx is erythematous and swollen and protrudes into the pharyngeal lumen. There is a diffuse maculopapular rash over her thighs and abdomen. Which of these surface structures interacts with the causative agent of her condition?
A. Constant part of TCR α-chain
B. CD4
C. CD3
D. CD1
E. Variable part of TCR β-chain (Correct Answer)
Explanation: ***Variable part of TCR β-chain***
- The clinical presentation suggests **toxic shock syndrome (TSS)**, likely caused by **Staphylococcus aureus** or **Streptococcus pyogenes**, which produce **superantigens** (e.g., toxic shock syndrome toxin-1, streptococcal pyrogenic exotoxins).
- Superantigens bind to the **variable beta chain (Vβ) of the T-cell receptor (TCR)** and the **MHC class II molecule** on antigen-presenting cells **outside the antigen-binding groove**, bypassing normal antigen processing.
- This leads to **massive, non-specific T-cell activation** (up to 20% of T cells) and **cytokine storm** (IL-2, IFN-γ, TNF-α), resulting in the clinical features of TSS: fever, hypotension, diffuse rash, and multiorgan dysfunction.
*Constant part of TCR α-chain*
- The **constant region of the TCR α-chain** is involved in structural integrity and signal transduction but does not directly interact with superantigens.
- Superantigens specifically target the **variable β chain** for cross-linking with MHC class II, not the constant α-chain.
*CD4*
- **CD4** is a co-receptor found on helper T cells that binds to **MHC class II molecules** during normal antigen presentation.
- While superantigens interact with MHC class II, the primary TCR interaction site is the **Vβ region**, not the CD4 co-receptor itself.
- CD4 plays a supportive role in TCR signaling but is not the direct binding target of superantigens.
*CD3*
- The **CD3 complex** (CD3γ, CD3δ, CD3ε, CD3ζ chains) associates with the TCR to form the complete **TCR-CD3 complex** and is essential for signal transduction upon antigen recognition.
- While CD3 is crucial for T-cell activation signaling, it is not the direct binding site for superantigens, which specifically engage the **Vβ region**.
*CD1*
- **CD1 molecules** are MHC class I-like glycoproteins that present **lipid and glycolipid antigens** (not peptide antigens) to specialized T cells, including NKT cells.
- CD1 is unrelated to the mechanism of superantigen binding, which involves **MHC class II** and the **variable beta chain of the TCR**.
Question 68: A 32-year-old man with hypertension and gout comes to the physician with left flank pain and bloody urine for two days. He does not smoke cigarettes but drinks two beers daily. Home medications include hydrochlorothiazide and ibuprofen as needed for pain. Physical examination shows left costovertebral angle tenderness. Urine dipstick is strongly positive for blood. Microscopic analysis of a stone found in the urine reveals a composition of magnesium ammonium phosphate. Which of the following is the strongest predisposing factor for this patient's condition?
A. Uric acid precipitation
B. Hereditary deficiency in amino acid reabsorption
C. Use of vitamin C supplements
D. Urinary tract infection (Correct Answer)
E. Ethylene glycol ingestion
Explanation: ***Urinary tract infection***
- The presence of a **magnesium ammonium phosphate stone**, also known as a **struvite stone**, is highly indicative of a urinary tract infection (UTI). These stones are formed in the presence of **urease-producing bacteria** (e.g., *Proteus mirabilis, Klebsiella pneumoniae*) that metabolize urea into ammonia, increasing localized pH and promoting crystal formation.
- While the patient has multiple predisposing factors for other types of kidney stones (e.g., gout and hydrochlorothiazide for calcium and uric acid stones), the specific composition of the stone points directly to an underlying infection as the primary predisposing factor.
*Uric acid precipitation*
- This typically leads to **uric acid stones**, which are common in patients with **gout** and **hyperuricemia**. The stone described is magnesium ammonium phosphate, not uric acid.
- Uric acid stones are usually radiolucent and are not directly associated with infection, unlike struvite stones.
*Hereditary deficiency in amino acid reabsorption*
- This condition, such as **cystinuria**, leads to the formation of **cystine stones**. These stones are hexagonal in shape and are caused by impaired tubular reabsorption of certain amino acids.
- The described stone composition is magnesium ammonium phosphate, not cystine.
*Use of vitamin C supplements*
- Excessive vitamin C (ascorbic acid) intake can be metabolized to **oxalate**, potentially contributing to **calcium oxalate stone** formation, especially in susceptible individuals.
- This patient's stone is a struvite stone, not a calcium oxalate stone.
*Ethylene glycol ingestion*
- Ethylene glycol poisoning is associated with the formation of **calcium oxalate monohydrate crystals** in the urine, which can lead to acute kidney injury and flank pain.
- The stone composition here is magnesium ammonium phosphate, indicating a different etiology.
Question 69: A 72-year-old patient presents to the emergency department because of abdominal pain, diarrhea, and fever. He was started on levofloxacin for community-acquired pneumonia 2 weeks prior with resolution of his pulmonary symptoms. He has had hypertension for 20 years, for which he takes amlodipine. His temperature is 38.3°C (101.0°F), pulse is 90/min, and blood pressure is 110/70 mm Hg. On examination, mild abdominal distension with minimal tenderness was found. Laboratory tests reveal a peripheral white blood cell count of 12.000/mm3 and a stool guaiac mildly positive for occult blood. Which of the following best describe the mechanism of this patient illness?
A. Damage to the gastrointestinal tract by enteropathogenic viruses
B. Autoimmune inflammation of the rectum
C. Disruption of normal bowel flora and infection by spore-forming rods (Correct Answer)
D. Decreased blood flow to the gastrointestinal tract
E. Presence of osmotically active, poorly absorbed solutes in the bowel lumen
Explanation: ***Disruption of normal bowel flora and infection by spore-forming rods***
- This describes **Clostridioides difficile infection (CDI)**, which is strongly suggested by the patient's recent antibiotic use (levofloxacin, a fluoroquinolone) followed by abdominal pain, diarrhea, fever, and leukocytosis.
- Antibiotics disrupt the normal gut microbiome, allowing **C. difficile (spore-forming rods)** to proliferate and produce toxins that cause colitis.
*Damage to the gastrointestinal tract by enteropathogenic viruses*
- While viral gastroenteritis can cause these symptoms, the **recent history of antibiotic use** makes CDI a much more likely diagnosis.
- Viral infections typically resolve spontaneously and are less likely to cause a significant **leukocytosis** and **occult blood in stool** in this context.
*Autoimmune inflammation of the rectum*
- Conditions like **ulcerative colitis**, an autoimmune disease, can cause similar symptoms but typically have a **chronic or relapsing course** and are not usually triggered by recent antibiotic use.
- The acute presentation following antibiotics strongly points away from an autoimmune process.
*Decreased blood flow to the gastrointestinal tract*
- **Ischemic colitis** can cause abdominal pain and bloody diarrhea, especially in older patients with vascular risk factors (like hypertension).
- However, the prominent **fever** and **leukocytosis**, coupled with recent antibiotic use, are more indicative of an infectious process like CDI than ischemia.
*Presence of osmotically active, poorly absorbed solutes in the bowel lumen*
- This mechanism describes **osmotic diarrhea**, which can be caused by malabsorption (e.g., lactose intolerance) or certain laxatives.
- Osmotic diarrhea typically **resolves with fasting** and is not usually associated with fever, significant leukocytosis, or occult blood in the stool, which are present here.
Question 70: A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1°F (38.9°C), pulse is 56/min, respirations are 16/min, and blood pressure is 150/85 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show:
Hemoglobin 13.5 g/dL
Leukocyte count 15,000/mm3
Platelet count 130,000/mm3
Serum
Na+ 129 mEq/L
Cl- 100 mEq/L
K+ 4.6 mEq/L
HCO3- 22 mEq/L
Urea nitrogen 14 mg/dL
Creatinine 1.3 mg/dL
An x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?
A. Urine antigen assay (Correct Answer)
B. CT Chest
C. Direct immunofluorescent antibody test
D. Stool culture
E. Polymerase chain reaction
Explanation: ***Urine antigen assay***
- This patient presents with **pneumonia symptoms** (low-grade fever, dry cough, dyspnea, bilateral infiltrates) along with **gastrointestinal symptoms** (watery diarrhea) and **hyponatremia**, after attending a hotel meeting with other sick attendees. These are classic features of **Legionnaires' disease**.
- A **urine antigen assay** is a rapid and highly specific test for **Legionella pneumophila serogroup 1**, which causes the majority of Legionnaires' disease cases.
*CT Chest*
- A CT scan of the chest would provide more detailed imaging of the lung infiltrates but is typically used to characterize findings once pneumonia is diagnosed or to rule out other lung pathologies, not as an initial diagnostic test for the specific pathogen.
- While it can reveal characteristic patterns, it doesn't identify the causative organism and is not the most appropriate *next step in diagnosis* for a presumed Legionella infection.
*Direct immunofluorescent antibody test*
- A **direct immunofluorescent antibody (DFA) test** is used to identify legionella in respiratory secretions. However, collecting a sufficiently good sputum sample can be difficult, especially with a **dry cough**.
- Its sensitivity is lower than urine antigen testing for serogroup 1 and requires a respiratory sample, making it less convenient for initial diagnosis.
*Stool culture*
- While the patient has diarrhea, a **stool culture** would primarily detect typical bacterial enteric pathogens (e.g., Salmonella, Shigella, Campylobacter) and would not identify **Legionella**.
- The diarrhea, in this context, is likely an extrapulmonary manifestation of Legionnaires' disease caused by Legionella, not a separate primary enteric infection.
*Polymerase chain reaction*
- **PCR testing** can detect Legionella DNA in respiratory samples, offering high sensitivity and specificity.
- However, it is generally less rapid and widely available than the urine antigen test for initial diagnosis of Legionella pneumophila serogroup 1, which is the most common cause of Legionnaires' disease.
