A 40-year-old man presents to the office complaining of chills, fever, and productive cough for the past 24 hours. He has a history of smoking since he was 18 years old. His vitals are: heart rate of 85/min, respiratory rate of 20/min, temperature 39.0°C (102.2°F), blood pressure 110/70 mm Hg. On physical examination, there is dullness on percussion on the upper right lobe, as well as bronchial breath sounds and egophony. The plain radiograph reveals an increase in density with an alveolar pattern in the upper right lobe. Which one is the most common etiologic agent of the suspected disease?
Q92
A 65-year-old male with a history of COPD presents to the emergency department with dyspnea, productive cough, and a fever of 40.0°C (104.0°F) for the past 2 days. His respiratory rate is 20/min, blood pressure is 125/85 mm Hg, and heart rate is 95/min. A chest X-ray is obtained and shows a right lower lobe infiltrate. Sputum cultures are pending and he is started on antibiotics. The patient has not received any vaccinations in the last 20 years. The physician discusses with him the importance of getting a vaccine that can produce immunity via which of the following mechanisms?
Q93
One week after starting amoxicillin for sinusitis, a 4-year-old girl is brought to the emergency department with fever, rash, and myalgia. She has been hospitalized multiple times for recurrent streptococcal pneumonia and meningitis. She appears tired. Examination shows a diffuse urticarial rash. Her antibiotic is discontinued. Which of the following is the most likely underlying mechanism for her recurrent infections?
Q94
A 32-year-old man is brought to the physician by his wife for a 3-day history of fever, headaches, and myalgias. He returned from a camping trip in Oklahoma 10 days ago. He works as a computer salesman. His temperature is 38.1°C (100.6°F). Neurologic examination shows a sustained clonus of the right ankle following sudden passive dorsiflexion. He is disoriented to place and time but recognizes his wife. Laboratory studies show a leukocyte count of 1,700/mm3 and a platelet count of 46,000/mm3. A peripheral blood smear shows monocytes with intracytoplasmic morulae. Which of the following is the most likely causal organism?
Q95
A 7-month-old Caucasian male presents with recurrent sinusitis and pharyngitis. The parents say that the child has had these symptoms multiple times in the past couple of months and a throat swab sample reveals the presence of Streptococcus pneumoniae. Upon workup for immunodeficiency it is noted that serum levels of immunoglobulins are extremely low but T-cell levels are normal. Which of the following molecules is present on the cells that this patient lacks?
Q96
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?
Q97
A 35-year-old man from Thailand presents with low-grade fever, chronic cough, and night sweats for 3 months. He describes the cough as productive and producing white sputum that is sometimes streaked with blood. He also says he has lost 10 lb in the last 3 months. Past medical history is unremarkable. The patient denies any smoking history, alcohol, or recreational drug use. The vital signs include blood pressure 115/75 mm Hg, heart rate 120/min, respiratory rate 20/min, and temperature 36.6℃ (97.8℉). On physical examination, the patient is ill-looking and thin with no pallor or jaundice. Cardiopulmonary auscultation reveals some fine crackles in the right upper lobe. A chest radiograph reveals a right upper lobe homogeneous density. Which of the following tests would be most helpful in making a definitive diagnosis of active infection in this patient?
Q98
A 46-year-old woman presents to her primary care physician with complaints of increasing left upper quadrant discomfort. She has a known history of type 1 Gaucher disease. On physical examination, her spleen is palpable 8 cm below the costal margin. Routine laboratory work reveals severe pancytopenia. After consultation with the patient on the risks of her condition, the patient decides to undergo a splenectomy. Which of the following is more likely to occur as a consequence of splenectomy in this patient?
Q99
A 45-year-old man visits the office with complaints of severe pain with urination for 5 days. In addition, he reports having burning discomfort and itchiness at the tip of his penis. He is also concerned regarding a yellow-colored urethral discharge that started a week ago. Before his symptoms began, he states that he had sexual intercourse with multiple partners at different parties organized by the hotel he was staying at. Physical examination shows edema and erythema concentrated around the urethral meatus accompanied by a mucopurulent discharge. Which of the following diagnostic tools will best aid in the identification of the causative agent for his symptoms?
Q100
A 7-month-old girl is brought to the hospital by her mother, who complains of a lesion on the infant’s labia for the past 5 days. The lesion is 2 x 2 cm in size and red in color with serosanguinous fluid oozing out of the right labia. The parents note that the girl has had a history of recurrent bacterial skin infections with no pus but delayed healing since birth. She also had delayed sloughing of the umbilical cord at birth. Complete blood count results are as follows:
Neutrophils on admission
Leukocytes 19,000/mm3
Neutrophils 83%
Lymphocytes 10%
Eosinophils 1%
Basophils 1%
Monocytes 5%
Hemoglobin 14 g/dL
Which of the following compounds is most likely to be deficient in this patient?
Bacteria US Medical PG Practice Questions and MCQs
Question 91: A 40-year-old man presents to the office complaining of chills, fever, and productive cough for the past 24 hours. He has a history of smoking since he was 18 years old. His vitals are: heart rate of 85/min, respiratory rate of 20/min, temperature 39.0°C (102.2°F), blood pressure 110/70 mm Hg. On physical examination, there is dullness on percussion on the upper right lobe, as well as bronchial breath sounds and egophony. The plain radiograph reveals an increase in density with an alveolar pattern in the upper right lobe. Which one is the most common etiologic agent of the suspected disease?
A. Legionella pneumophila
B. Chlamydia pneumoniae
C. Mycoplasma pneumoniae
D. Streptococcus pneumoniae (Correct Answer)
E. Haemophilus influenzae
Explanation: ***Streptococcus pneumoniae***
- This patient presents with classic symptoms of **community-acquired pneumonia (CAP)**, including fever, chills, productive cough, and specific findings on physical exam (dullness, bronchial breath sounds, egophony) and chest X-ray (**lobar consolidation**).
- **_Streptococcus pneumoniae_** is the most common bacterial cause of CAP worldwide, accounting for a significant percentage of cases, especially in adults.
*Legionella pneumophila*
- While _Legionella_ can cause severe pneumonia, it often presents with **GI symptoms** (diarrhea, nausea) and **neurological symptoms** (confusion) in addition to respiratory symptoms, which are not described here.
- Risk factors typically include exposure to **contaminated water sources**, and the pneumonia can be rapidly progressive.
*Chlamydia pneumoniae*
- _Chlamydia pneumoniae_ typically causes a more **atypical pneumonia**, often with a more insidious onset, prolonged cough, and less severe systemic symptoms.
- It usually presents as a **walking pneumonia** with milder findings on chest X-ray, unlike the clear lobar consolidation described.
*Mycoplasma pneumoniae*
- Like _Chlamydia pneumoniae_, _Mycoplasma pneumoniae_ is a common cause of **atypical pneumonia**, often with a gradual onset, hacking cough, and less pronounced fever.
- It rarely causes the classic lobar consolidation seen in this patient and is often referred to as "walking pneumonia."
*Haemophilus influenzae*
- _Haemophilus influenzae_ is a significant cause of CAP, especially in patients with **underlying lung disease** (like COPD) or other comorbidities.
- While certainly a possibility given the patient's smoking history, **_Streptococcus pneumoniae_** remains the overall most common cause of bacterial CAP in otherwise healthy adults.
Question 92: A 65-year-old male with a history of COPD presents to the emergency department with dyspnea, productive cough, and a fever of 40.0°C (104.0°F) for the past 2 days. His respiratory rate is 20/min, blood pressure is 125/85 mm Hg, and heart rate is 95/min. A chest X-ray is obtained and shows a right lower lobe infiltrate. Sputum cultures are pending and he is started on antibiotics. The patient has not received any vaccinations in the last 20 years. The physician discusses with him the importance of getting a vaccine that can produce immunity via which of the following mechanisms?
A. T cell-dependent B cell response (Correct Answer)
B. Natural killer cell response
C. Mast cell degranulation response
D. Complement activation response
E. T cell-independent B cell response
Explanation: ***T cell-dependent B cell response***
- The patient, suffering from **community-acquired pneumonia** frequently caused by *Streptococcus pneumoniae*, would benefit from the **pneumococcal polysaccharide conjugate vaccine (PCV13)**.
- PCV13 contains **polysaccharide antigens conjugated to a protein carrier**, which allows for a **T cell-dependent B cell response**, leading to better immunogenicity, **memory cell formation**, and broader protection, especially in adults.
*Natural killer cell response*
- **Natural killer (NK) cells** are part of the **innate immune system** and are primarily involved in surveillance against infected or cancerous cells, not primary antibody production for bacterial infections.
- While NK cells play a role in overall immunity, they are not the primary mechanism by which **vaccines induce long-term protective immunity** against encapsulated bacteria like *S. pneumoniae*.
*Mast cell degranulation response*
- **Mast cell degranulation** is primarily associated with **allergic reactions** and defense against parasites, releasing mediators like histamine.
- This mechanism is not involved in generating protective immunity against bacterial infections through vaccination; rather, it can be a harmful immune response in certain contexts.
*Complement activation response*
- **Complement activation** is a crucial part of both innate and adaptive immunity, aiding in pathogen clearance through opsonization, lysis, and inflammation.
- While antibodies produced by vaccination can activate complement, the vaccine itself doesn't directly induce immunity via complement; rather, complement acts downstream of the **antibody response**.
*T cell-independent B cell response*
- The **pneumococcal polysaccharide vaccine (PPSV23)**, made from **unconjugated polysaccharides**, induces a T cell-independent B cell response, which is less robust in adults and does not produce memory cells.
- This response is characterized by poorer immunogenicity in young children and elderly individuals, and a lack of **affinity maturation** or **immunological memory** compared to T cell-dependent responses.
Question 93: One week after starting amoxicillin for sinusitis, a 4-year-old girl is brought to the emergency department with fever, rash, and myalgia. She has been hospitalized multiple times for recurrent streptococcal pneumonia and meningitis. She appears tired. Examination shows a diffuse urticarial rash. Her antibiotic is discontinued. Which of the following is the most likely underlying mechanism for her recurrent infections?
A. Impaired opsonization (Correct Answer)
B. Absence of IgA antibodies
C. Defective superoxide production
D. Impaired leukocyte adhesion
E. Accumulation of bradykinin
Explanation: ***Impaired opsonization***
- Recurrent severe bacterial infections, particularly with **encapsulated organisms** like *Streptococcus pneumoniae*, suggest a defect in the **complement system** or **antibody production**, which are crucial for opsonization.
- The pattern of recurrent **pneumonia and meningitis** with *Streptococcus pneumoniae* is highly suggestive of **complement deficiency** (particularly C3 or alternative pathway defects), which impairs opsonization and clearance of encapsulated bacteria.
*Absence of IgA antibodies*
- Selective IgA deficiency typically presents with recurrent **mucosal infections** (e.g., upper respiratory, gastrointestinal), but usually not severe, disseminated bacterial infections like meningitis.
- While IgA is important for mucosal immunity, its absence alone does not explain generalized recurrent **streptococcal pneumonia** and **meningitis** as the primary underlying cause.
*Defective superoxide production*
- Defective superoxide production, as seen in **chronic granulomatous disease (CGD)**, leads to recurrent infections with **catalase-positive organisms** (e.g., *Staphylococcus aureus*, *Aspergillus*).
- The patient's history of recurrent **streptococcal infections** (which are catalase-negative) and lack of **granuloma formation** makes CGD less likely.
*Impaired leukocyte adhesion*
- Impaired leukocyte adhesion, characteristic of **leukocyte adhesion deficiency (LAD)**, leads to recurrent bacterial infections, **impaired wound healing**, and **delayed umbilical cord separation**.
- While recurrent infections are present, the clinical picture does not specifically mention impaired wound healing or delayed cord separation, and the specific organisms involved (streptococcal) are not pathognomonic for LAD.
*Accumulation of bradykinin*
- Accumulation of bradykinin leads to **hereditary angioedema**, characterized by recurrent episodes of localized **swelling** without urticaria.
- This condition does not primarily manifest with recurrent bacterial infections or a diffuse urticarial rash in response to medication, making it an unlikely underlying mechanism for the patient's symptoms.
Question 94: A 32-year-old man is brought to the physician by his wife for a 3-day history of fever, headaches, and myalgias. He returned from a camping trip in Oklahoma 10 days ago. He works as a computer salesman. His temperature is 38.1°C (100.6°F). Neurologic examination shows a sustained clonus of the right ankle following sudden passive dorsiflexion. He is disoriented to place and time but recognizes his wife. Laboratory studies show a leukocyte count of 1,700/mm3 and a platelet count of 46,000/mm3. A peripheral blood smear shows monocytes with intracytoplasmic morulae. Which of the following is the most likely causal organism?
A. Coxiella burnetii
B. Rickettsia rickettsii
C. Anaplasma phagocytophilum
D. Borrelia burgdorferi
E. Ehrlichia chaffeensis (Correct Answer)
Explanation: ***Correct: Ehrlichia chaffeensis***
- The presence of **intracytoplasmic morulae** in **monocytes** is a pathognomonic sign for *Ehrlichia chaffeensis* infection, which causes **human monocytic ehrlichiosis**.
- The patient's symptoms of **fever, headache, myalgias, thrombocytopenia, leukopenia**, and the history of a **camping trip** in an endemic area (Oklahoma) are highly consistent with ehrlichiosis.
*Incorrect: Coxiella burnetii*
- This bacterium causes **Q fever**, characterized by fever, headache, and atypical pneumonia, but it does **not cause intracytoplasmic morulae** in monocytes or frequently lead to the degree of leukopenia and thrombocytopenia seen here.
- While it can be acquired from environments, the **microscopic findings** rule it out in this specific case.
*Incorrect: Rickettsia rickettsii*
- This organism causes **Rocky Mountain spotted fever**, which presents with fever, headache, myalgias, and a characteristic **rash** that is usually present on the palms and soles, none of which are mentioned here.
- It does not form **intracytoplasmic morulae** in monocytes.
*Incorrect: Anaplasma phagocytophilum*
- This bacterium causes **human granulocytic anaplasmosis**, which is clinically similar to ehrlichiosis but forms **morulae in granulocytes** (neutrophils), not monocytes.
- The peripheral blood smear specifically identifies morulae in **monocytes**, directing towards *Ehrlichia*.
*Incorrect: Borrelia burgdorferi*
- This spirochete causes **Lyme disease**, characterized by an **expanding erythematous rash (erythema migrans)**, fever, and musculoskeletal symptoms, but it does not cause leukopenia or thrombocytopenia.
- It does not produce **morulae** in any blood cells.
Question 95: A 7-month-old Caucasian male presents with recurrent sinusitis and pharyngitis. The parents say that the child has had these symptoms multiple times in the past couple of months and a throat swab sample reveals the presence of Streptococcus pneumoniae. Upon workup for immunodeficiency it is noted that serum levels of immunoglobulins are extremely low but T-cell levels are normal. Which of the following molecules is present on the cells that this patient lacks?
A. CD8
B. CD3
C. CD4
D. NKG2D
E. CD19 (Correct Answer)
Explanation: ***CD19***
- The patient's symptoms of **recurrent bacterial infections** (sinusitis, pharyngitis with *Streptococcus pneumoniae*) and **extremely low serum immunoglobulin levels** despite normal T-cell levels are highly indicative of **X-linked agammaglobulinemia (XLA)**.
- XLA is characterized by the absence of **mature B cells**, which are responsible for antibody production. **CD19** is a pan B-cell marker, universally expressed on B cells from their earliest stages in the bone marrow (pro-B cells) through to mature circulating B cells, identifying the cell population that is lacking in this patient.
*CD8*
- **CD8** is a co-receptor expressed on **cytotoxic T cells**, involved in recognizing antigens presented by MHC class I molecules.
- The patient's T-cell levels are reported as **normal**, indicating that both CD4+ and CD8+ T-cell populations are likely present.
*CD3*
- **CD3** is a complex of proteins expressed on the surface of all **T lymphocytes** (both helper and cytotoxic T cells) as part of the T-cell receptor (TCR) complex.
- Since the patient has **normal T-cell levels**, cells expressing CD3 are present and functional.
*CD4*
- **CD4** is a co-receptor primarily expressed on **helper T cells**, which recognize antigens presented by MHC class II molecules.
- Similar to CD8 and CD3, **normal T-cell levels** suggest that CD4+ T cells are present in this patient.
*NKG2D*
- **NKG2D** is an activating receptor expressed primarily on **NK cells** (natural killer cells), as well as on subsets of CD8+ T cells and gamma-delta T cells.
- While NK cells play a role in innate immunity, their absence or dysfunction would not directly explain the specific deficiency of **immunoglobulins** and recurrent bacterial infections seen here, which points to a B-cell defect.
Question 96: 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 97: A 35-year-old man from Thailand presents with low-grade fever, chronic cough, and night sweats for 3 months. He describes the cough as productive and producing white sputum that is sometimes streaked with blood. He also says he has lost 10 lb in the last 3 months. Past medical history is unremarkable. The patient denies any smoking history, alcohol, or recreational drug use. The vital signs include blood pressure 115/75 mm Hg, heart rate 120/min, respiratory rate 20/min, and temperature 36.6℃ (97.8℉). On physical examination, the patient is ill-looking and thin with no pallor or jaundice. Cardiopulmonary auscultation reveals some fine crackles in the right upper lobe. A chest radiograph reveals a right upper lobe homogeneous density. Which of the following tests would be most helpful in making a definitive diagnosis of active infection in this patient?
A. PPD test
B. Silver stain
C. Ziehl-Neelsen stain (Correct Answer)
D. Gram stain
E. Interferon-gamma assay
Explanation: ***Ziehl-Neelsen stain***
- The patient's symptoms (low-grade fever, chronic cough with white/bloody sputum, night sweats, weight loss) and chest X-ray findings (right upper lobe homogeneous density) are highly suggestive of **active tuberculosis (TB)**, especially given his origin from Thailand (a country with a high TB burden).
- The **Ziehl-Neelsen stain** (acid-fast stain) directly visualizes **acid-fast bacilli** (AFB) like *Mycobacterium tuberculosis* in sputum, providing a rapid and definitive diagnosis of active infection.
*PPD test*
- A **PPD test** (tuberculin skin test) indicates exposure to *Mycobacterium tuberculosis* but cannot differentiate between **latent TB infection** and **active disease**.
- A positive PPD can occur in individuals previously exposed or vaccinated with BCG, offering no direct evidence of current active infection.
*Silver stain*
- **Silver stain** (e.g., Gomori methenamine silver) is used to identify fungal organisms like *Pneumocystis jirovecii* or certain bacteria, such as *Legionella pneumophila*.
- It is not the primary stain for diagnosing tuberculosis, which requires detection of acid-fast bacilli.
*Gram stain*
- **Gram stain** is used to classify bacteria based on their cell wall properties (Gram-positive or Gram-negative).
- *Mycobacterium tuberculosis* has a unique **mycolic acid-rich cell wall** that makes it resistant to Gram staining and requires acid-fast staining for visualization.
*Interferon-gamma assay*
- An **interferon-gamma release assay (IGRA)**, like the Quantiferon-TB Gold test, detects exposure to *Mycobacterium tuberculosis* and is used to diagnose **latent TB infection**.
- Similar to the PPD test, it cannot distinguish between latent infection and **active disease**, and a positive result requires further investigation for active TB.
Question 98: A 46-year-old woman presents to her primary care physician with complaints of increasing left upper quadrant discomfort. She has a known history of type 1 Gaucher disease. On physical examination, her spleen is palpable 8 cm below the costal margin. Routine laboratory work reveals severe pancytopenia. After consultation with the patient on the risks of her condition, the patient decides to undergo a splenectomy. Which of the following is more likely to occur as a consequence of splenectomy in this patient?
A. Thrombocytopenia
B. Leukopenia
C. Pneumococcal septicemia (Correct Answer)
D. Staphylococcal septicemia
E. Anemia
Explanation: ***Pneumococcal septicemia***
- Patients who undergo splenectomy are at significantly increased risk of **overwhelming post-splenectomy infection (OPSI)**, particularly from **encapsulated bacteria** like *Streptococcus pneumoniae*.
- The spleen plays a crucial role in filtering encapsulated bacteria and producing opsonizing antibodies, and its removal compromises this immune function.
*Thrombocytopenia*
- **Thrombocytopenia** is typically a symptom *before* splenectomy in Gaucher disease due to hypersplenism.
- After splenectomy, the platelet count often **increases**, not decreases, due to the removal of the organ that sequesters platelets and destroys them.
*Leukopenia*
- **Leukopenia** (specifically neutropenia) is a pre-existing condition in severe Gaucher disease due to hypersplenism and bone marrow involvement.
- Post-splenectomy, the white blood cell count, particularly neutrophils, generally **increases** as the sequestration and destruction in the spleen are eliminated.
*Staphylococcal septicemia*
- While *Staphylococcus* can cause serious infections, it is **not the primary pathogen** associated with OPSI in asplenic patients.
- Encapsulated bacteria like *Streptococcus pneumoniae* are the most common and dangerous cause of post-splenectomy sepsis.
*Anemia*
- **Anemia** is a common finding in Gaucher disease due to hypersplenism and bone marrow infiltration.
- Splenectomy typically **improves** anemia by removing the site of red blood cell destruction and reducing abnormal cytokine production that inhibits erythropoiesis.
Question 99: A 45-year-old man visits the office with complaints of severe pain with urination for 5 days. In addition, he reports having burning discomfort and itchiness at the tip of his penis. He is also concerned regarding a yellow-colored urethral discharge that started a week ago. Before his symptoms began, he states that he had sexual intercourse with multiple partners at different parties organized by the hotel he was staying at. Physical examination shows edema and erythema concentrated around the urethral meatus accompanied by a mucopurulent discharge. Which of the following diagnostic tools will best aid in the identification of the causative agent for his symptoms?
A. Nucleic acid amplification tests (NAATs) (Correct Answer)
B. Urethral biopsy
C. Tzanck smear
D. Leukocyte esterase dipstick test
E. Gram stain
Explanation: ***Nucleic acid amplification tests (NAATs)***
- NAATs are the **most sensitive and specific diagnostic tools** for detecting common sexually transmitted infections (STIs) like **gonorrhea** and **chlamydia**, which present with urethral discharge, dysuria, and itching.
- They can identify the **genetic material** of the causative organisms directly from urine samples or urethral swabs, making them highly effective even with low bacterial loads.
*Urethral biopsy*
- A urethral biopsy is an **invasive procedure** generally reserved for investigating conditions like **strictures, tumors, or chronic inflammatory diseases** when other diagnostic methods are inconclusive.
- It is not a primary diagnostic tool for acute urethritis suspected to be an STI, as it carries risks and is unnecessary given the availability of less invasive options.
*Tzanck smear*
- The Tzanck smear is primarily used for diagnosing **herpes simplex virus (HSV) infections** by looking for multinucleated giant cells and intranuclear inclusions.
- While HSV can cause genital lesions, it typically does not present as a primary symptom of mucopurulent urethral discharge and dysuria without visible vesicles or ulcers, making it less likely in this scenario.
*Leukocyte esterase dipstick test*
- A leukocyte esterase dipstick test detects the presence of **white blood cells** in urine, indicating inflammation or infection in the urinary tract.
- While it can suggest urethritis, it is **not specific for the causative agent** and merely indicates inflammation, requiring further specific testing to identify the pathogen.
*Gram stain*
- A Gram stain of urethral discharge can rapidly identify Gram-negative intracellular diplococci suggestive of **gonorrhea** (Neisseria gonorrhoeae).
- However, its sensitivity for gonorrhea is lower than NAATs, especially in asymptomatic cases or for detecting other common causes of urethritis like **Chlamydia trachomatis**, which are not visible on Gram stain.
Question 100: A 7-month-old girl is brought to the hospital by her mother, who complains of a lesion on the infant’s labia for the past 5 days. The lesion is 2 x 2 cm in size and red in color with serosanguinous fluid oozing out of the right labia. The parents note that the girl has had a history of recurrent bacterial skin infections with no pus but delayed healing since birth. She also had delayed sloughing of the umbilical cord at birth. Complete blood count results are as follows:
Neutrophils on admission
Leukocytes 19,000/mm3
Neutrophils 83%
Lymphocytes 10%
Eosinophils 1%
Basophils 1%
Monocytes 5%
Hemoglobin 14 g/dL
Which of the following compounds is most likely to be deficient in this patient?
A. Selectin
B. TNF-alpha
C. Cellular adhesion molecule
D. Integrin subunit (Correct Answer)
E. vWF
Explanation: **Integrin subunit** ✓
- The patient exhibits features consistent with **Leukocyte Adhesion Deficiency Type 1 (LAD-1)**, characterized by recurrent bacterial infections without pus formation, delayed umbilical cord separation, and **leukocytosis with neutrophil predominance** (due to inability of neutrophils to exit circulation and migrate to tissues).
- LAD-1 is caused by a defect in the **CD18 integrin β2 subunit**, which is essential for the function of β2 integrins (e.g., LFA-1, MAC-1, CR3) involved in firm adhesion and transmigration of leukocytes across the endothelium.
- The absence of pus despite bacterial infections is explained by the inability of neutrophils to migrate to infection sites.
*Selectin*
- **Selectins** (E-selectin, P-selectin, L-selectin) mediate the initial **rolling** phase of leukocyte adhesion to the endothelium.
- Defects in selectins (specifically fucose metabolism) are associated with **LAD-2 (Leukocyte Adhesion Deficiency Type 2)**, which presents with similar infection susceptibility but typically includes additional features like growth retardation, intellectual disability, and the Bombay blood phenotype.
*TNF-alpha*
- **TNF-alpha** is a pro-inflammatory cytokine that upregulates adhesion molecules on endothelial cells and activates immune responses.
- While TNF-alpha deficiency or blockade can increase infection susceptibility, it does not explain the specific constellation of delayed umbilical cord separation, persistent neutrophilic leukocytosis, and infections without pus formation seen in this patient.
*Cellular adhesion molecule*
- This is a broad, non-specific term encompassing multiple adhesion molecule families (integrins, selectins, immunoglobulin superfamily members, cadherins).
- While integrins are indeed cellular adhesion molecules, **"integrin subunit"** is the specific and precise answer for LAD-1, as the defect involves the β2 integrin subunit (CD18).
*vWF*
- **Von Willebrand Factor (vWF)** is crucial for platelet adhesion to damaged endothelium and serves as a carrier protein for factor VIII.
- Deficiency causes **Von Willebrand disease**, presenting with mucocutaneous bleeding (epistaxis, easy bruising, menorrhagia), not recurrent bacterial infections, delayed wound healing, or the immunologic abnormalities observed in this patient.
