A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?
Q2
An investigator inoculates three different broths with one colony-forming unit of Escherichia coli. Broth A contains 100 μmol of lactose, broth B contains 100 μmol of glucose, and broth C contains both 100 μmol of lactose and 100 μmol of glucose. After 24 hours, the amounts of lactose, galactose, and glucose in the three broths are measured. The results of the experiment are shown:
Lactose Galactose Glucose
Broth A 43 μmol 11 μmol 9 μmol
Broth B 0 μmol 0 μmol 39 μmol
Broth C 94 μmol 1 μmol 66 μmol
The observed results are most likely due to which of the following properties of broth A compared to broth C?
Q3
Part of the success of the Streptococcus pyogenes bacterium lies in its ability to evade phagocytosis. Which of the following helps in this evasion?
Q4
A scientist is studying the influenza A virus. He focuses on two strains – one from humans (H7N1) and one from horses (H3N8). He takes cells from chickens and coinfects these cells with both influenza strains. From these chicken cells, the scientist isolates a new strain and finds that this new strain can infect human cells. He further characterizes the new strain’s hemagglutinin and neuraminidase description as H7N8. What term best describes the process that underlies these experimental results?
Q5
A 35-year-old female presents to the emergency room complaining of diarrhea and dehydration. She has been experiencing severe watery diarrhea for the past 3 days. She reports that she has been unable to leave the bathroom for more than a few minutes at a time. The diarrhea is profuse and watery without visible blood or mucus. She recently returned from a volunteer trip to Yemen where she worked at an orphanage. Her past medical history is notable for psoriasis for which she takes sulfasalazine. The patient drinks socially and does not smoke. Her temperature is 99°F (37.2°C), blood pressure is 100/55 mmHg, pulse is 130/min, and respirations are 20/min. Mucous membranes are dry. Her eyes appear sunken. Capillary refill is 4 seconds. The patient is started on intravenous fluid resuscitation. Which of the following processes is capable of transmitting the genetic material for the toxin responsible for this patient's condition?
Q6
An investigator is studying DNA repair processes in an experimental animal. The investigator inactivates a gene encoding a protein that physiologically excises nucleotides from damaged, bulky, helix-distorting DNA strands. A patient with a similar defect in this gene is most likely to present with which of the following findings?
Q7
An 8-year-old child is brought to the emergency department because of profuse diarrhea and vomiting that have lasted for 2 days. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. The family recently made a trip to India to visit relatives. Today, his heart rate is 100/min, respiratory rate is 22/min, blood pressure is 105/65 mm Hg, and temperature is 37.2ºC (99.0°F). On physical examination, he appears unwell with poor skin turgor and dry oral mucosa. His heart has a regular rate and rhythm and his lungs are clear to auscultation bilaterally. His abdomen is sensitive to shallow and deep palpation. A gross examination of the stool reveals a 'rice water' appearance. Diagnostic microbiology results are pending. Which of the following is the best diagnostic test to aid in the identification of this patient's condition?
Q8
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?
Q9
A 2-year-old boy has a history of recurrent bacterial infections, especially of his skin. When he has an infection, pus does not form. His mother reports that, when he was born, his umbilical cord took 5 weeks to detach. He is ultimately diagnosed with a defect in a molecule in the pathway that results in neutrophil extravasation. Which of the following correctly pairs the defective molecule with the step of extravasation that molecule affects?
Q10
A 42-year-old woman presents to a medical office with complaints of fatigue, weight loss, and low-grade fever for 1 week. She noticed bleeding spots on her feet this morning. The past medical history is significant for a recent dental appointment. She is a non-smoker and does not drink alcohol. She does not currently take any medications. On examination, the vital signs include temperature 37.8°C (100.0°F), blood pressure 138/90 mm Hg, respirations 21/min, and pulse 87/min. Cardiac auscultation reveals a pansystolic murmur in the mitral area with radiation to the left axilla. Laboratory studies show hemoglobin levels of 17.2 g/dL, erythrocyte sedimentation rate (ESR) of 25 mm/h, and a white blood cell (WBC) count of 12,000 cells/mm3. An echocardiogram reveals valvular vegetations on the mitral valve with mild regurgitation. Blood samples are sent for bacterial culture. Empiric antibiotic therapy is initiated with ceftriaxone and vancomycin. The blood cultures most likely will yield the growth of which of the following organisms?
Bacterial genetics US Medical PG Practice Questions and MCQs
Question 1: A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?
A. CUA (Correct Answer)
B. CCC
C. UAA
D. CCA
E. AUG
Explanation: ***CUA***
- The **wobble hypothesis** allows for non-standard base pairing at the **third position** of the codon.
- Since CUU codes for leucine, a change in the third base to **A (CUA)** can often still code for the same amino acid due to degeneracy of the genetic code.
*CCC*
- This codon codes for **proline**, not leucine.
- A change in the **second letter** of the codon almost always results in a different amino acid.
*UAA*
- This is one of the **stop codons** (UAA, UAG, UGA), which signals the termination of translation.
- It does not code for any amino acid.
*CCA*
- This codon codes for **proline**, not leucine.
- Changing the first or second nucleotide typically results in a different amino acid.
*AUG*
- This codon codes for **methionine** and also serves as the **start codon** for protein synthesis.
- It does not code for leucine.
Question 2: An investigator inoculates three different broths with one colony-forming unit of Escherichia coli. Broth A contains 100 μmol of lactose, broth B contains 100 μmol of glucose, and broth C contains both 100 μmol of lactose and 100 μmol of glucose. After 24 hours, the amounts of lactose, galactose, and glucose in the three broths are measured. The results of the experiment are shown:
Lactose Galactose Glucose
Broth A 43 μmol 11 μmol 9 μmol
Broth B 0 μmol 0 μmol 39 μmol
Broth C 94 μmol 1 μmol 66 μmol
The observed results are most likely due to which of the following properties of broth A compared to broth C?
A. Absence of glucose preventing catabolite repression (Correct Answer)
B. Higher lactose concentration allowing increased metabolism
C. Lower pH favoring lactose utilization
D. Presence of additional nutrients enhancing growth
E. Reduced osmotic pressure facilitating enzyme activity
Explanation: ***Absence of glucose preventing catabolite repression***
- In Broth A, the absence of **glucose** means that **catabolite repression** does not occur, allowing the *E. coli* to immediately and more extensively utilize **lactose**.
- This leads to a greater breakdown of lactose into galactose and glucose, and subsequent further metabolism of these sugars, evidenced by the lower final lactose concentration and higher galactose/glucose conversion than in Broth C.
*Higher lactose concentration allowing increased metabolism*
- All broths initially contain **100 μmol of lactose**, so there is no higher initial concentration in Broth A to explain the difference in metabolism.
- The observed difference is in the *extent* of lactose utilization, not in the initial amount available.
*Lower pH favoring lactose utilization*
- The experiment does not provide any information about the **pH levels** in the broths.
- While pH can affect enzyme activity, there is no basis in the given data to suggest a pH difference as the cause for varied lactose utilization.
*Presence of additional nutrients enhancing growth*
- All broths are inoculated with the same organism and the only specified difference is the sugar content (lactose, glucose, or both).
- There is no mention of **additional nutrients** being present in Broth A compared to Broth C.
*Reduced osmotic pressure facilitating enzyme activity*
- The initial total solute concentration in Broth A (100 μmol lactose) is similar to Broth B (100 μmol glucose) and lower than Broth C (100 μmol lactose + 100 μmol glucose).
- While **osmotic pressure** can affect bacterial growth, there is nothing in the data to suggest it is the primary factor explaining the difference in lactose metabolism between Broth A and Broth C.
Question 3: Part of the success of the Streptococcus pyogenes bacterium lies in its ability to evade phagocytosis. Which of the following helps in this evasion?
A. Streptolysin S
B. Streptolysin O
C. Streptokinase
D. M protein (Correct Answer)
E. Pyrogenic toxin
Explanation: ***M protein***
- The **M protein** is a major virulence factor of *Streptococcus pyogenes* that **inhibits phagocytosis** by binding to factor H, a host complement regulatory protein, preventing C3b deposition.
- It also helps the bacterium adhere to host cells and resist killing by neutrophils.
*Streptolysin S*
- **Streptolysin S** is a **hemolysin** that causes beta-hemolysis on blood agar and contributes to tissue damage by lysing cells.
- While contributing to virulence, its primary role is not direct inhibition of phagocytosis but rather cell lysis.
*Streptolysin O*
- **Streptolysin O (SLO)** is another **hemolysin** that produces pore-forming toxins, leading to cell lysis and tissue destruction.
- It is highly antigenic and often used as a diagnostic marker (ASO titer) for past *S. pyogenes* infections, but it does not directly prevent phagocytosis.
*Streptokinase*
- **Streptokinase** is an enzyme that activates plasminogen, leading to the breakdown of fibrin clots, which helps in the **spread of infection** within tissues.
- Its main function is not to evade phagocytosis but rather to facilitate invasion by dissolving blood clots that would typically wall off the infection.
*Pyrogenic toxin*
- **Pyrogenic toxins** (also known as erythrogenic toxins) are superantigens that cause symptoms like fever and rash (e.g., in scarlet fever) by stimulating a massive, non-specific T-cell activation.
- These toxins contribute to the systemic manifestations of infection but do not directly interfere with the process of phagocytosis.
Question 4: A scientist is studying the influenza A virus. He focuses on two strains – one from humans (H7N1) and one from horses (H3N8). He takes cells from chickens and coinfects these cells with both influenza strains. From these chicken cells, the scientist isolates a new strain and finds that this new strain can infect human cells. He further characterizes the new strain’s hemagglutinin and neuraminidase description as H7N8. What term best describes the process that underlies these experimental results?
A. Transduction
B. Antigenic drift
C. Transformation
D. Conjugation
E. Antigenic shift (Correct Answer)
Explanation: ***Antigenic shift***
- **Antigenic shift** in influenza viruses refers to the process where two different influenza strains **coinfect** the same host cell, leading to a **reassortment** of their segmented genomes.
- This reassortment creates a novel viral strain with a new combination of hemagglutinin (H) and neuraminidase (N) antigens (e.g., H7N1 + H3N8 → H7N8), which can lead to pandemics.
*Transduction*
- **Transduction** is a process where foreign DNA is introduced into a cell by a virus or viral vector, primarily seen in bacteria.
- It involves the transfer of genetic material via **bacteriophages** and does not describe the reassortment of influenza virus segments.
*Antigenic drift*
- **Antigenic drift** involves small, gradual changes in the H and N antigens of influenza viruses due to **point mutations** during replication.
- These slow mutations lead to seasonal epidemics, but not the creation of a completely new subtype as described.
*Transformation*
- **Transformation** is the process by which a cell takes up naked DNA from its environment, incorporating it into its own genome.
- This mechanism is common in bacteria for acquiring new genetic traits and is not applicable to the reassortment of viral segments within a coinfected host cell.
*Conjugation*
- **Conjugation** is a process of genetic material transfer between bacteria through direct cell-to-cell contact, typically via a **pilus**.
- This mechanism is distinct from viral genetic reassortment and does not involve the coinfection of a host cell by different viral strains.
Question 5: A 35-year-old female presents to the emergency room complaining of diarrhea and dehydration. She has been experiencing severe watery diarrhea for the past 3 days. She reports that she has been unable to leave the bathroom for more than a few minutes at a time. The diarrhea is profuse and watery without visible blood or mucus. She recently returned from a volunteer trip to Yemen where she worked at an orphanage. Her past medical history is notable for psoriasis for which she takes sulfasalazine. The patient drinks socially and does not smoke. Her temperature is 99°F (37.2°C), blood pressure is 100/55 mmHg, pulse is 130/min, and respirations are 20/min. Mucous membranes are dry. Her eyes appear sunken. Capillary refill is 4 seconds. The patient is started on intravenous fluid resuscitation. Which of the following processes is capable of transmitting the genetic material for the toxin responsible for this patient's condition?
A. Transposition
B. Conjugation
C. Endospore formation
D. Transduction (Correct Answer)
E. Transformation
Explanation: ***Transduction***
- The patient's symptoms are highly suggestive of **cholera**, caused by *Vibrio cholerae*, which produces **cholera toxin**.
- The genes for cholera toxin are carried on a **bacteriophage (CTXφ)**, and their transfer between bacteria occurs via **transduction**.
*Transposition*
- **Transposition** involves the movement of **transposons ("jumping genes")** within a genome or between DNA molecules.
- While transposons can carry antimicrobial resistance genes or virulence factors, this mechanism is not typically associated with the transfer of the primary cholera toxin genes.
*Conjugation*
- **Conjugation** is the transfer of genetic material between bacteria through direct cell-to-cell contact, often involving a **pilus** and the transfer of **plasmids**.
- While *Vibrio cholerae* can engage in conjugation, the cholera toxin genes are predominantly acquired via specialized transduction with the CTXφ phage, not typically plasmid-mediated conjugation.
*Endospore formation*
- **Endospore formation** is a survival mechanism used by certain bacteria (e.g., *Clostridium*, *Bacillus*) to withstand harsh environmental conditions.
- It is not a mechanism for **horizontal gene transfer** or the transmission of toxin-encoding genetic material between bacteria.
*Transformation*
- **Transformation** is the uptake of **naked DNA** from the environment by a bacterial cell.
- While *Vibrio cholerae* can be naturally competent for transformation, the cholera toxin genes are primarily acquired through **phage-mediated transduction**, not free DNA uptake.
Question 6: An investigator is studying DNA repair processes in an experimental animal. The investigator inactivates a gene encoding a protein that physiologically excises nucleotides from damaged, bulky, helix-distorting DNA strands. A patient with a similar defect in this gene is most likely to present with which of the following findings?
A. Ataxic gait and facial telangiectasias
B. Malignant breast and ovarian growths
C. Leukocoria and a painful bone mass
D. Colorectal and endometrial cancers
E. Dry skin and increased photosensitivity (Correct Answer)
Explanation: ***Dry skin and increased photosensitivity***
- The description of excising **nucleotides from damaged, bulky, helix-distorting DNA strands** points to a defect in **Nucleotide Excision Repair (NER)**.
- Patients with defects in NER, such as those with **xeroderma pigmentosum**, are highly susceptible to UV-induced DNA damage, leading to **dry skin, increased photosensitivity**, and a high risk of skin cancers.
*Ataxic gait and facial telangiectasias*
- This constellation of symptoms is characteristic of **ataxia-telangiectasia**, a disorder caused by mutations in the **ATM gene**, which is involved in **DNA double-strand break repair**.
- While a DNA repair defect, it's not primarily linked to the excision of bulky, helix-distorting DNA strands.
*Malignant breast and ovarian growths*
- These cancers are commonly associated with mutations in the **BRCA1 and BRCA2 genes**, which play crucial roles in **homologous recombination repair of DNA double-strand breaks**.
- This type of repair is distinct from the excision of bulky, helix-distorting DNA strands described in the question.
*Leukocoria and a painful bone mass*
- **Leukocoria** can indicate **retinoblastoma**, linked to mutations in the **RB1 tumor suppressor gene**, which regulates the cell cycle but isn't primarily a DNA repair gene.
- A painful bone mass could suggest **osteosarcoma**, which is sometimes seen in retinoblastoma patients but not directly related to the specific DNA repair defect described.
*Colorectal and endometrial cancers*
- These cancers are hallmarks of **Lynch syndrome (hereditary nonpolyposis colorectal cancer - HNPCC)**, which is caused by defects in **Mismatch Repair (MMR)** genes (e.g., MLH1, MSH2, MSH6, PMS2).
- Mismatch repair corrects errors that arise during DNA replication, which is different from excising bulky, helix-distorting DNA damage.
Question 7: An 8-year-old child is brought to the emergency department because of profuse diarrhea and vomiting that have lasted for 2 days. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. The family recently made a trip to India to visit relatives. Today, his heart rate is 100/min, respiratory rate is 22/min, blood pressure is 105/65 mm Hg, and temperature is 37.2ºC (99.0°F). On physical examination, he appears unwell with poor skin turgor and dry oral mucosa. His heart has a regular rate and rhythm and his lungs are clear to auscultation bilaterally. His abdomen is sensitive to shallow and deep palpation. A gross examination of the stool reveals a 'rice water' appearance. Diagnostic microbiology results are pending. Which of the following is the best diagnostic test to aid in the identification of this patient's condition?
A. Methylene blue wet mount
B. Gram stain of stool sample
C. Dark-field microscopy
D. Rapid diagnostic test for cholera toxin
E. Stool culture on TCBS agar (Correct Answer)
Explanation: ***Stool culture on TCBS agar***
- The patient's symptoms (profuse watery diarrhea, vomiting, dehydration, history of travel to India) strongly suggest **cholera**, caused by *Vibrio cholerae*.
- **Thiosulfate-citrate-bile salts-sucrose (TCBS) agar** is a highly selective medium specifically used for isolating *Vibrio* species.
*Methylene blue wet mount*
- This test is primarily used to identify **white blood cells (leukocytes)** in stool, which indicate an inflammatory process, such as in *Shigella* or *Salmonella* infections.
- Cholera is a **non-inflammatory** diarrhea, so a methylene blue wet mount would likely be negative for leukocytes and therefore not helpful for diagnosis.
*Gram stain of stool sample*
- While Gram stain can classify bacteria, it is generally **not useful for diagnosing diarrheal diseases** caused by specific enteric pathogens, as stool contains a vast array of Gram-negative and Gram-positive bacteria.
- It would be difficult to identify *Vibrio cholerae* among the normal flora using this method alone.
*Dark-field microscopy*
- This technique is typically used to visualize **spirochetes**, such as *Treponema pallidum* (syphilis), due to their characteristic motility and morphology.
- While *Vibrio cholerae* are motile rods, dark-field microscopy is **not the standard or most sensitive method** for its identification in a stool sample, especially compared to selective cultures.
*Rapid diagnostic test for cholera toxin*
- While such tests exist and can be useful in epidemic settings for quick screening, they generally have **lower sensitivity and specificity** compared to culture-based methods.
- **Culture remains the gold standard** for definitive diagnosis, especially for guiding treatment and epidemiological surveillance.
Question 8: 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 9: A 2-year-old boy has a history of recurrent bacterial infections, especially of his skin. When he has an infection, pus does not form. His mother reports that, when he was born, his umbilical cord took 5 weeks to detach. He is ultimately diagnosed with a defect in a molecule in the pathway that results in neutrophil extravasation. Which of the following correctly pairs the defective molecule with the step of extravasation that molecule affects?
A. E-selectin; transmigration
B. LFA-1 (integrin); tight adhesion (Correct Answer)
C. ICAM-1; margination
D. E-selectin; tight adhesion
E. PECAM-1; transmigration
Explanation: ***LFA-1 (integrin); tight adhesion***
- This patient's symptoms (recurrent bacterial infections, lack of pus formation, and delayed umbilical cord separation) are classic for **Leukocyte Adhesion Deficiency type 1 (LAD-1)**.
- **LAD-1** is caused by a defect in the **CD18 subunit** of **β2 integrins**, including **LFA-1** and **Mac-1**, which are crucial for the **tight adhesion** of neutrophils to endothelial cells.
*E-selectin; transmigration*
- **E-selectin** mediates the initial **rolling** of leukocytes along the endothelial surface, not transmigration.
- A defect in E-selectin would impair rolling, but the primary defect in LAD-1 is in tight adhesion.
*ICAM-1; margination*
- **ICAM-1** (Intercellular Adhesion Molecule-1) is an endothelial ligand that binds to integrins on leukocytes, facilitating **tight adhesion** and transmigration, not margination.
- **Margination** refers to the movement of leukocytes to the periphery of the blood vessel lumen.
*E-selectin; tight adhesion*
- **E-selectin** is involved in the initial **rolling** phase of extravasation by binding to sialyl Lewis X on leukocytes.
- It does not primarily mediate **tight adhesion**, which is facilitated by integrins binding to ICAM-1.
*PECAM-1; transmigration*
- **PECAM-1** (Platelet Endothelial Cell Adhesion Molecule-1) is primarily involved in **transmigration** (diapedesis), where leukocytes pass between endothelial cells.
- While important for extravasation, the characteristic findings of LAD-1 point to a defect earlier in the pathway, specifically tight adhesion.
Question 10: A 42-year-old woman presents to a medical office with complaints of fatigue, weight loss, and low-grade fever for 1 week. She noticed bleeding spots on her feet this morning. The past medical history is significant for a recent dental appointment. She is a non-smoker and does not drink alcohol. She does not currently take any medications. On examination, the vital signs include temperature 37.8°C (100.0°F), blood pressure 138/90 mm Hg, respirations 21/min, and pulse 87/min. Cardiac auscultation reveals a pansystolic murmur in the mitral area with radiation to the left axilla. Laboratory studies show hemoglobin levels of 17.2 g/dL, erythrocyte sedimentation rate (ESR) of 25 mm/h, and a white blood cell (WBC) count of 12,000 cells/mm3. An echocardiogram reveals valvular vegetations on the mitral valve with mild regurgitation. Blood samples are sent for bacterial culture. Empiric antibiotic therapy is initiated with ceftriaxone and vancomycin. The blood cultures most likely will yield the growth of which of the following organisms?
A. Actinomyces israelii
B. Coxiella burnetii
C. Group B Streptococcus
D. Staphylococcus aureus
E. Streptococcus viridans (Correct Answer)
Explanation: ***Streptococcus viridans***
- The patient's recent **dental appointment**, the presence of an **oral organism**, and symptoms consistent with **subacute bacterial endocarditis** (fatigue, fever, weight loss, petechiae) strongly suggest *S. viridans* as the causative agent.
- *Streptococcus viridans* species are common inhabitants of the **oral flora** and are a leading cause of endocarditis following dental procedures, especially in individuals with pre-existing valvular abnormalities.
*Actinomyces israelii*
- While *Actinomyces israelii* is an oral commensal, it typically causes **actinomycosis**, a chronic, suppurative infection characterized by abscess formation and sinus tracts, often following dental procedures.
- It less commonly presents as acute or subacute endocarditis and would not typically cause the rapid progression of symptoms described.
*Coxiella burnetii*
- *Coxiella burnetii* causes **Q fever**, which can manifest as endocarditis, often associated with exposure to **farm animals** or their products.
- The patient's history lacks any such exposure, and the clinical presentation is more aligned with standard bacterial endocarditis from oral flora.
*Group B Streptococcus*
- **Group B Streptococcus** (*Streptococcus agalactiae*) is primarily known as a cause of **neonatal sepsis** and meningitis, and infections in immunocompromised adults or those with underlying conditions like diabetes.
- It is not typically associated with endocarditis following a dental procedure in an otherwise healthy adult.
*Staphylococcus aureus*
- **Staphylococcus aureus** can cause endocarditis, particularly in **intravenous drug users** or patients with prosthetic valves/indwelling lines, and often presents as a more **acute and aggressive disease**.
- While possible, the association with a recent dental procedure and the subacute course makes *Streptococcus viridans* a more likely culprit in this specific scenario.
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