Infectious Diseases Practice Questions

Q: A 25-year-old woman presents with a severe sore throat, fever, and difficulty swallowing. Examination reveals swollen tonsils with white exudate and tender cervical lymphadenopathy. What is the most likely diagnosis?

Streptococcal pharyngitis. Streptococcal pharyngitis - The combination of severe sore throat, fever, swollen tonsils with white exudate, and tender cervical lymphadenopathy is highly suggestive of Group A Streptococcus (GAS) infection. - This clinical presentation aligns with the Centor criteria for diagnosing strep throat, which includes tonsillar exudates, tender anterior cervical lymph nodes, absence of cough, and history of fever. Viral pharyngitis - While viral pharyngitis can cause sore throat and fever, it often presents with additional symptoms like cough, rhinorrhea (runny nose), and conjunctivitis, which are absent here. - White exudates on the tonsils are less common and typically not as prominent in viral cases compared to bacterial infections. Epstein-Barr virus infection - Infectious mononucleosis due to EBV can present with severe pharyngitis, exudative tonsillitis, fever, and lymphadenopathy, but it often includes fatigue, splenomegaly, and atypical lymphocytosis, which are not mentioned in this case. - While it's a possibility, the classic presentation without other distinguishing features makes streptococcal pharyngitis more likely. Diphtheria - Diphtheria is characterized by a greyish-white pseudomembrane that is firmly attached to the tonsils and pharynx and bleeds if scraped, which is not described [1]. - This disease is now rare due to vaccination and typically involves more systemic toxicity and potential airway obstruction [1].

Q: A 40-year-old man who recently traveled to South America presents with a chronic, ulcerative lesion on his arm. A biopsy reveals amastigotes within macrophages. What is the most likely diagnosis?

Cutaneous leishmaniasis. ### Cutaneous leishmaniasis - The patient's recent travel to **South America**, a region endemic for Leishmaniasis, combined with a chronic, **ulcerative lesion** and the presence of **amastigotes within macrophages** on biopsy, are classic diagnostic features [1]. - Amastigotes are the non-motile, intracellular form of Leishmania parasites found in human tissues [1]. *Cutaneous tuberculosis* - While it can manifest as chronic ulcerative lesions, the biopsy would show **granulomas with caseation necrosis** containing acid-fast bacilli, not amastigotes within macrophages. - Diagnosis typically involves isolation of *Mycobacterium tuberculosis* or molecular tests, which is not described. *Sporotrichosis* - Characterized by **nodular lesions** that can ulcerate and spread along lymphatic channels, often associated with a history of **thorn prick** or contact with decaying vegetation. - Biopsy would reveal **cigar-shaped budding yeasts** (Sporothrix schenckii), not amastigotes within macrophages. *Mycetoma* - Presents as a chronic, destructive infection primarily affecting the **feet**, characterized by **swelling, sinuses, and grains** (macroscopic aggregates of microorganisms) being discharged. - Histology would show **granulomas with characteristic grains** (fungal hyphae or bacterial filaments), significantly different from amastigotes.

Q: A 28-year-old man presents with fever, chills, and a rash that began on the wrists and ankles and spread to the trunk. What is the most likely diagnosis?

Rocky Mountain spotted fever. ***Rocky Mountain spotted fever*** - The classic presentation involves **fever, chills**, and a **rash** that starts on the **wrists and ankles** and spreads centrally to the trunk [1]. - This is a **tick-borne illness** caused by *Rickettsia rickettsii*, a highly suggestive clinical picture [1]. *Lyme disease* - Characterized by an expanding **erythema migrans** (bull's-eye rash), which is typically large and annular, not a diffuse rash spreading from extremities. - While it can cause fever and chills, the rash morphology and spread pattern are inconsistent with this case description. *Measles* - Presents with a **maculopapular rash** that typically starts on the **face** and spreads downwards to the trunk and extremities, not initiating on the wrists and ankles. - Associated with the "3 Cs": **cough, coryza, and conjunctivitis**, and possibly **Koplik spots**, which are not mentioned. *Meningococcemia* - Often presents with an abrupt onset of **fever, headache, stiff neck**, and a **petechial or purpuric rash**, which does not typically start on the wrists and ankles. - The rash is usually diffuse and can rapidly progress to **purpura fulminans**, often indicating severe sepsis.

Q: A patient presents with diarrhea, bloating, and weight loss. Stool analysis shows pear-shaped, flagellated protozoa. What is the most likely diagnosis?

Giardiasis. Giardiasis - The classic presentation of diarrhea, bloating, and weight loss combined with the microscopic finding of pear-shaped, flagellated protozoa [1] in stool is pathognomonic for Giardia lamblia infection [1]. - Giardia trophozoites are easily identifiable by their distinctive morphology [1] and motility in fresh stool preparations. Amebiasis - Caused by Entamoeba histolytica, which typically presents with bloody diarrhea (dysentery) [2] and abdominal pain. - Trophozoites are larger, amoeboid, and lack flagella; they may contain ingested red blood cells. Ascariasis - This is an infection by a roundworm (Ascaris lumbricoides), a multicellular parasite, not a single-celled protozoan. - Symptoms can include abdominal pain, malnutrition, or pulmonary involvement during larval migration, but pear-shaped flagellated protozoa would not be seen. Schistosomiasis - Caused by a fluke (Schistosoma species), a type of flatworm, which is also a multicellular parasite, not a protozoan. - Symptoms vary depending on the species and stage of infection but often involve urinary or intestinal inflammation and are diagnosed by finding eggs (not flagellated protozoa) in stool or urine.

Q: Which test is most appropriate for diagnosing Clostridium difficile infection in a patient presenting with diarrhea after antibiotic use?

Stool PCR for C. difficile toxin. ***Stool PCR for C. difficile toxin*** - A **stool PCR test** is highly sensitive and specific for detecting the presence of the *C. difficile* pathogen and/or its **toxin genes**, making it the most appropriate and rapid diagnostic method [2]. - This test identifies the **DNA of *C. difficile*** and/or its toxin genes (**tcdA** and **tcdB**), which are responsible for the severe diarrhea and colitis [2]. *Stool culture* - While stool culture can identify *C. difficile*, it has **lower specificity** because asymptomatic carriers can also harbor the bacteria [1]. - Culture is also **time-consuming** (2-3 days), which can delay crucial treatment for *C. difficile* infection. *CT scan of the abdomen* - A **CT scan** is not a primary diagnostic tool for *C. difficile* infection but can be used to assess for complications like **toxic megacolon** or **colonic perforation** in severe cases [1]. - It does not directly identify the presence of the pathogen or its toxins, so it's not the initial diagnostic test for the infection itself. *Serum electrolytes* - **Serum electrolytes** are important for monitoring the patient's hydration status and detecting imbalances caused by severe diarrhea (e.g., **hypokalemia**, **hyponatremia**). - However, they do not diagnose the underlying cause of the diarrhea, which in this case is suspected *C. difficile* infection.

Q: Which organ system is primarily affected by the yellow fever virus, leading to its characteristic symptoms?

Liver. ***Liver*** - The **yellow fever virus** primarily targets and replicates within **hepatocytes**, leading to **hepatic necrosis** and dysfunction. - This liver damage causes the characteristic **jaundice** (yellowing of skin and eyes) due to impaired bilirubin metabolism. *Lungs* - While respiratory symptoms can occur in severe cases due to systemic inflammation or secondary infections, the **lungs are not the primary target organ** of the yellow fever virus. - **Pneumonia** or **acute respiratory distress syndrome (ARDS)** may develop as complications, but not as the initial and defining feature of the disease. *Kidneys* - **Kidney failure** can be a severe complication of yellow fever, often due to **hypotension**, **hemorrhage**, and **multi-organ dysfunction syndrome**. - However, the kidneys are **secondarily affected** by the systemic illness rather than being the primary site of viral replication and damage. *Heart* - Myocardial involvement, such as **myocarditis**, can occur in severe yellow fever cases and contribute to **cardiac dysfunction** and **shock**. - Similar to the kidneys, the heart is typically **not the primary organ** directly targeted by the virus, but rather affected by the systemic inflammatory response and organ damage.

Q: A 32-year-old woman is advised on post-exposure prophylaxis (PEP) following a needlestick injury. What is the recommended regimen for HIV PEP?

Three-drug regimen. ***Three-drug regimen*** - A **three-drug regimen** is the **standard recommendation** for effective HIV post-exposure prophylaxis (PEP) to maximize the chances of preventing infection. - This approach typically involves a combination of two **nucleoside reverse transcriptase inhibitors (NRTIs)** and one **integrase strand transfer inhibitor (INSTI)** or a **protease inhibitor (PI)** [1]. *Single drug regimen* - A **single-drug regimen** is generally **insufficient** for preventing HIV infection after a significant exposure due to the risk of viral resistance and lower efficacy. - This approach would **not meet current guidelines** for optimal post-exposure prophylaxis. *Two-drug regimen* - While a **two-drug regimen** might be considered in very low-risk exposures or in situations where a three-drug regimen is contraindicated, it is **not the preferred standard**. - The **efficacy is lower** compared to a three-drug regimen, especially when the source virus's resistance profile is unknown or the exposure risk is moderate to high [1]. *Four-drug regimen* - A **four-drug regimen** is **not routinely recommended** for HIV PEP as there is no evidence that adding a fourth drug significantly improves efficacy over a three-drug regimen. - It would increase the risk of **drug-related toxicities and side effects** without additional benefit in most cases.

Q: What is the most common cause of sepsis?

Bacterial infections. ***Bacterial infections*** - The vast majority of sepsis cases are triggered by **bacterial infections**, with common culprits including *Staphylococcus aureus*, *Escherichia coli*, and *Streptococcus pneumoniae* [1]. - These bacteria can originate from various sites such as the lungs (pneumonia), urinary tract (urosepsis), abdomen, and skin [1]. *Viral infections* - While viral infections can sometimes lead to sepsis, they are a **less common cause** than bacterial infections. - Examples include severe influenza or COVID-19, but they typically lead to a septic response much less frequently than bacteria. *Fungal infections* - Fungal infections, particularly *Candida* species, can cause sepsis, especially in **immunocompromised individuals** or those with prolonged hospitalization [1]. - However, they are significantly less common as a primary cause of sepsis compared to bacteria. *Parasitic infections* - Parasitic infections are a **rare cause of sepsis** in most developed regions, though opportunistic infections can occur in immunocompromised patients [1]. - Examples like severe malaria can lead to a systemic inflammatory response, but they are not among the most common global causes of sepsis [1].

Q: A 30-year-old woman with recent travel to Southeast Asia presents with an eschar at the site of a chigger bite and a febrile illness. What is the most appropriate initial treatment?

Doxycycline. ***Doxycycline*** - This presentation is highly suggestive of **scrub typhus**, an infectious disease endemic to Southeast Asia, characterized by a **febrile illness** and an **eschar** at the site of a chigger bite. - **Doxycycline** is the drug of choice for treating scrub typhus, as well as other rickettsial infections, due to its efficacy against *Orientia tsutsugamushi*. *Azithromycin* - While azithromycin can be used as an alternative for scrub typhus, particularly in pregnant women or young children, it is **less effective** than doxycycline, especially in severe cases. - It is typically considered when doxycycline is contraindicated. *Ciprofloxacin* - Ciprofloxacin is a **fluoroquinolone antibiotic** that is generally not effective against *Orientia tsutsugamushi* or other rickettsial infections. - It targets bacterial DNA gyrase and topoisomerase IV, mechanisms that are not crucial for eradicating this specific pathogen. *Amoxicillin* - **Amoxicillin** is a penicillin-class antibiotic that is ineffective against atypical bacteria like *Orientia tsutsugamushi* because it targets bacterial cell wall synthesis which is not a primary mechanism of action against rickettsial organisms. - It would not be an appropriate treatment for scrub typhus.

Q: A 25-year-old woman presents with fever, rash, and joint pain after returning from Southeast Asia. What is the most likely diagnosis?

Dengue fever. ***Dengue fever*** - The classic triad of **fever**, **rash**, and **arthralgia** (joint pain) following travel to endemic areas like Southeast Asia is highly suggestive of dengue [1]. - Dengue is a **mosquito-borne viral infection** common in tropical and subtropical regions. *Malaria* - Characterized by **cyclic fevers**, chills, and sweats [2], but rash and prominent joint pain are not typical features. - Diagnosed by **microscopic examination** of blood smears for parasites or rapid diagnostic tests. *Typhoid fever* - Presents with a **prolonged fever**, headache, abdominal pain, and often a **"rose spot" rash**, but significant joint pain is uncommon. - Caused by **Salmonella Typhi** and transmitted through contaminated food or water. *Leptospirosis* - Typically causes **fever**, headache, muscle pain (myalgia), and sometimes jaundice, but a widespread rash and prominent arthralgia are less characteristic. - Transmitted through contact with **urine of infected animals** or contaminated water.

Question 1

A 25-year-old woman presents with a severe sore throat, fever, and difficulty swallowing. Examination reveals swollen tonsils with white exudate and tender cervical lymphadenopathy. What is the most likely diagnosis?

Accepted Answer

Streptococcal pharyngitis

Answer

Viral pharyngitis

Answer

Epstein-Barr virus infection

Answer

Diphtheria

Question 2

A 40-year-old man who recently traveled to South America presents with a chronic, ulcerative lesion on his arm. A biopsy reveals amastigotes within macrophages. What is the most likely diagnosis?

Accepted Answer

Cutaneous leishmaniasis

Answer

Cutaneous tuberculosis

Answer

Sporotrichosis

Answer

Mycetoma

Question 3

A 28-year-old man presents with fever, chills, and a rash that began on the wrists and ankles and spread to the trunk. What is the most likely diagnosis?

Accepted Answer

Rocky Mountain spotted fever

Answer

Lyme disease

Answer

Measles

Answer

Meningococcemia

Question 4

A patient presents with diarrhea, bloating, and weight loss. Stool analysis shows pear-shaped, flagellated protozoa. What is the most likely diagnosis?

Accepted Answer

Giardiasis

Answer

Amebiasis

Answer

Ascariasis

Answer

Schistosomiasis

Question 5

Which test is most appropriate for diagnosing Clostridium difficile infection in a patient presenting with diarrhea after antibiotic use?

Accepted Answer

Stool PCR for C. difficile toxin

Answer

Stool culture

Answer

CT scan of the abdomen

Answer

Serum electrolytes

Question 6

Which organ system is primarily affected by the yellow fever virus, leading to its characteristic symptoms?

Accepted Answer

Liver

Answer

Lungs

Answer

Kidneys

Answer

Heart

Question 7

A 32-year-old woman is advised on post-exposure prophylaxis (PEP) following a needlestick injury. What is the recommended regimen for HIV PEP?

Accepted Answer

Three-drug regimen

Answer

Single drug regimen

Answer

Two-drug regimen

Answer

Four-drug regimen

Question 8

What is the most common cause of sepsis?

Accepted Answer

Bacterial infections

Answer

Viral infections

Answer

Fungal infections

Answer

Parasitic infections

Question 9

A 30-year-old woman with recent travel to Southeast Asia presents with an eschar at the site of a chigger bite and a febrile illness. What is the most appropriate initial treatment?

Accepted Answer

Doxycycline

Answer

Azithromycin

Answer

Ciprofloxacin

Answer

Amoxicillin

Question 10

A 25-year-old woman presents with fever, rash, and joint pain after returning from Southeast Asia. What is the most likely diagnosis?

Accepted Answer

Dengue fever

Answer

Malaria

Answer

Typhoid fever

Answer

Leptospirosis

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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