Infectious Diseases — MCQs
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A 45-year-old man with AIDS presents with productive cough, fever, and night sweats. Chest X-ray reveals an ill-defined area of consolidation at the periphery of the right middle lobe and mediastinal lymphadenopathy. Sputum culture yields acid-fast bacilli. The patient subsequently develops severe headache and neck rigidity. Which of the following brain areas is most likely affected by this patient's infection?
Which of the following is NOT true about cutaneous anthrax?
Regarding Schick's test, which of the following is false?
An inoculum size of 10-100 bacteria or cysts can produce infection in all except:
An increased frequency of infections due to which of the following organisms has been reported among patients with achlorhydria?
Skin testing with tuberculin-PPD is most widely used in screening for:
Which human infection spreads through urine?
Which disease is transmitted by mites?
A patient presents with fever for 3 weeks. On examination, splenomegaly is observed. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?
A young lady presents with symptoms of Urinary Tract Infection. All of the following findings on a midstream urine sample support the diagnosis of 'Uncomplicated Acute Cystitis', EXCEPT?
Infectious Diseases Indian Medical PG Practice Questions and MCQs
Question 31: A 45-year-old man with AIDS presents with productive cough, fever, and night sweats. Chest X-ray reveals an ill-defined area of consolidation at the periphery of the right middle lobe and mediastinal lymphadenopathy. Sputum culture yields acid-fast bacilli. The patient subsequently develops severe headache and neck rigidity. Which of the following brain areas is most likely affected by this patient's infection?
- A. Base of the brain (Correct Answer)
- B. Cerebellum
- C. Hippocampus
- D. Periventricular white matter
Explanation: **Explanation:** The clinical presentation of productive cough, fever, night sweats, and acid-fast bacilli in sputum in an immunocompromised patient (AIDS) strongly suggests **Tuberculosis (TB)**. The subsequent development of headache and neck rigidity indicates **Tuberculous Meningitis (TBM)**, a common extrapulmonary complication in HIV-infected individuals. **1. Why "Base of the brain" is correct:** Tuberculous meningitis is characterized by a thick, gelatinous **exudate** that preferentially accumulates in the **basal cisterns** (interpeduncular fossa, pontine cistern, and around the optic chiasm). This basal distribution occurs because the bacilli spread via the subarachnoid space after the rupture of a subependymal tubercle (Rich focus). This predilection for the base of the brain often leads to cranial nerve palsies (CN III, IV, and VI) and obstructive hydrocephalus. **2. Why other options are incorrect:** * **Cerebellum:** While tuberculomas can occur here, it is not the primary site for the characteristic meningeal exudate. * **Hippocampus:** Classically associated with **Herpes Simplex Virus (HSV-1) encephalitis** or Rabies (Negri bodies), not TB. * **Periventricular white matter:** This is the characteristic site for **Progressive Multifocal Leukoencephalopathy (PML)** caused by the JC virus or Primary CNS Lymphoma in AIDS patients. **Clinical Pearls for NEET-PG:** * **Rich Focus:** A small tuberculous lesion in the brain parenchyma or meninges that ruptures into the subarachnoid space to cause TBM. * **CSF Findings in TBM:** High protein, very low glucose, and **lymphocytic pleocytosis** (Cobweb coagulum formation). * **Imaging:** Contrast-enhanced CT/MRI typically shows **basal meningeal enhancement**. * **Complication:** The most common cranial nerve involved in TBM is the **6th Nerve (Abducens)**.
Question 32: Which of the following is NOT true about cutaneous anthrax?
- A. It is extremely painful. (Correct Answer)
- B. The entire affected area is congested and edematous.
- C. It presents with central crustation and a black eschar.
- D. Satellite nodules may appear around the inguinal region.
Explanation: Cutaneous anthrax, caused by *Bacillus anthracis*, is the most common clinical form of the disease. Understanding its classic presentation is vital for NEET-PG. **Why Option A is the correct answer (The "Not True" statement):** The hallmark of a cutaneous anthrax lesion is that it is **painless**. Despite its angry, necrotic appearance, the lesion lacks significant pain or tenderness. If a similar-looking lesion is exquisitely painful, a clinician should instead suspect a staphylococcal carbuncle or orf. **Analysis of other options:** * **Option B:** The lesion is typically surrounded by extensive **non-pitting edema** and congestion. This is due to the "Edema Factor" (EF), a component of the anthrax toxin that increases cAMP levels, leading to massive fluid accumulation in the extracellular space. * **Option C:** The lesion begins as a papule, progresses to a vesicle, and eventually undergoes central necrosis to form a characteristic **painless black eschar** (malignant pustule) with a surrounding rim of edema. * **Option D:** While the primary lesion is localized, **satellite vesicles** or nodules may occasionally develop. Regional lymphadenopathy (e.g., inguinal or axillary, depending on the site of inoculation) is common and may be associated with systemic symptoms. **High-Yield Clinical Pearls for NEET-PG:** 1. **Occupational Hazard:** Often called "Hide-porter’s disease" because it affects those handling infected animal products. 2. **Microscopy:** *B. anthracis* shows a "Bamboo stick" appearance and "Medusa head" colonies on agar. 3. **McFadyean’s Reaction:** Used to visualize the capsule (polypeptide of D-glutamic acid) using polychrome methylene blue. 4. **String of Pearls Reaction:** Occurs when grown on agar containing penicillin.
Question 33: Regarding Schick's test, which of the following is false?
- A. An erythematous reaction in both arms indicates an allergic type interpreted as Schick type.
- B. A positive test means that the person is immune to hypersensitivity diphtheria. (Correct Answer)
- C. Diphtheria antitoxin is given intradermally.
- D. All of the above.
Explanation: The **Schick test** is an intradermal test used to determine the immune status of an individual against *Corynebacterium diphtheriae*. It assesses the presence of circulating antitoxin antibodies. ### **Explanation of the Correct Option** **Option B is False (Correct Answer):** A **positive test** (erythema and swelling at the toxin site) indicates that the person **lacks immunity** and is susceptible to diphtheria. It means there are insufficient antitoxin antibodies to neutralize the injected toxin. Conversely, a negative test indicates the person is immune. The term "hypersensitivity diphtheria" is also clinically inaccurate in this context. ### **Analysis of Other Options** * **Option A (True statement):** If an erythematous reaction occurs in both the test and control arms and fades within 48 hours, it is a **Pseudo-reaction**. This indicates the individual is immune to the toxin but hypersensitive to the bacterial proteins (allergic type). * **Option C (True statement):** The test involves the intradermal injection of 0.1 ml of purified Diphtheria toxin in one arm (test) and 0.1 ml of heat-inactivated toxin in the other (control). ### **High-Yield Clinical Pearls for NEET-PG** * **Interpretation Summary:** * **Positive:** Reaction only at the test site (Susceptible). * **Negative:** No reaction at either site (Immune). * **Pseudo-reaction:** Erythema at both sites, disappears by 48h (Immune + Hypersensitive). * **Combined reaction:** Both sites react, but the test site reaction persists longer (Susceptible + Hypersensitive). * **Current Status:** The Schick test is largely obsolete and has been replaced by **ELISA** to measure antitoxin titers. * **Dick Test:** Similar intradermal test used for Scarlet Fever (*Streptococcus pyogenes*).
Question 34: An inoculum size of 10-100 bacteria or cysts can produce infection in all except:
- A. Shigella
- B. Enterohemorrhagic Escherichia coli
- C. Giardia lamblia
- D. Vibrio cholerae (Correct Answer)
Explanation: The core concept behind this question is **Infectious Dose (ID₅₀)**, which refers to the number of organisms required to cause disease in 50% of exposed individuals. Organisms that are highly resistant to gastric acid require a very low inoculum, whereas acid-sensitive organisms require a high inoculum to survive the stomach's acidic environment. ### Why Vibrio cholerae is the Correct Answer: *Vibrio cholerae* is highly **acid-sensitive**. To cause infection, it must survive the gastric acid barrier to reach the small intestine. Consequently, it requires a very large inoculum—typically **10⁵ to 10⁸ organisms**—to produce clinical cholera. If gastric acidity is neutralized (e.g., by antacids or achlorhydria), the infectious dose drops significantly. ### Why the Other Options are Incorrect: * **Shigella (Option A):** Highly acid-resistant. It can cause disease with as few as **10–100 bacilli**, making it highly communicable via person-to-person contact. * **Enterohemorrhagic E. coli (EHEC/O157:H7) (Option B):** Similar to Shigella, EHEC has a very low infectious dose (**<100 organisms**), which explains why outbreaks can occur through minimally contaminated food or water. * **Giardia lamblia (Option C):** The cyst stage of this protozoan is resistant to environmental stressors and gastric acid. Ingestion of only **10–25 cysts** is sufficient to cause giardiasis. ### High-Yield Clinical Pearls for NEET-PG: * **Low Inoculum (<100):** *Shigella*, *EHEC*, *Giardia*, *Entamoeba histolytica*, and *Coxiella burnetii* (the latter requires only 1 organism). * **High Inoculum (>10⁵):** *Vibrio cholerae*, *Salmonella* (except *S. Typhi*, which is moderate), and *Campylobacter jejuni*. * **Clinical Correlation:** Patients on Proton Pump Inhibitors (PPIs) or those with gastrectomies are at a significantly higher risk for infections by acid-sensitive organisms like *Vibrio* and *Salmonella*.
Question 35: An increased frequency of infections due to which of the following organisms has been reported among patients with achlorhydria?
- A. Salmonella (Correct Answer)
- B. Shigella
- C. Entamoeba
- D. Vibrio cholera
Explanation: **Explanation:** The correct answer is **Salmonella (Option A)**. **Underlying Medical Concept:** Gastric acid (HCl) serves as a primary innate immune barrier against ingested pathogens. **Achlorhydria** (absence of gastric acid) or the use of proton pump inhibitors (PPIs) significantly lowers the infectious dose required for certain bacteria to cause disease. *Salmonella* species are particularly **acid-labile**, meaning they are easily destroyed by a normal gastric pH of 1.5–3.5. In patients with achlorhydria, these bacteria survive the passage through the stomach and reach the small intestine, leading to an increased frequency of infections like Enteric fever and Salmonellosis. **Analysis of Incorrect Options:** * **B. Shigella:** Unlike Salmonella, *Shigella* is highly **acid-resistant**. It can survive the gastric barrier even at normal acidity levels, requiring an extremely low infectious dose (as few as 10–100 organisms). Thus, its frequency is not significantly altered by achlorhydria. * **C. Entamoeba:** *Entamoeba histolytica* is transmitted via **cysts**, which are naturally resistant to gastric acid. The acid actually helps in the excystation process in the lower GI tract. * **D. Vibrio cholerae:** While *V. cholerae* is also acid-labile, the question specifically targets the organism most classically associated with increased frequency in clinical reports of achlorhydria. While achlorhydria *is* a risk factor for Cholera, *Salmonella* is the more frequent and high-yield association in medical literature and exams regarding this specific physiological deficit. **High-Yield Clinical Pearls for NEET-PG:** * **Acid-Labile Organisms:** *Salmonella*, *Vibrio cholerae*, and *Campylobacter*. * **Acid-Resistant Organisms:** *Shigella*, *Helicobacter pylori*, and *Hepatitis A virus*. * Conditions predisposing to these infections include long-term PPI use, gastrectomy, and pernicious anemia.
Question 36: Skin testing with tuberculin-PPD is most widely used in screening for:
- A. Active TB
- B. Latent M. tuberculosis infection (Correct Answer)
- C. Miliary TB
- D. Overwhelming TB
Explanation: ### Explanation **1. Why the Correct Answer is Right:** The Tuberculin Skin Test (TST) or Mantoux test is based on a **Type IV (Delayed-type) Hypersensitivity reaction**. When PPD (Purified Protein Derivative) is injected intradermally, it triggers sensitized T-cells in individuals previously exposed to *M. tuberculosis*. The primary clinical utility of the TST is to identify individuals with **Latent Tuberculosis Infection (LTBI)**—those who are infected but do not have active disease. In these patients, the immune system has successfully "contained" the bacilli, but memory T-cells remain, leading to a positive skin reaction. **2. Why the Incorrect Options are Wrong:** * **Active TB (Option A):** While a TST can be positive in active TB, it is **not** a diagnostic tool for it. Diagnosis of active TB requires clinical, radiological, and microbiological (Sputum AFB/CBNAAT) evidence. Furthermore, up to 25% of patients with active TB may show a false-negative TST due to "anergy." * **Miliary and Overwhelming TB (Options C & D):** In severe, disseminated forms of tuberculosis, the body’s cell-mediated immunity is often profoundly suppressed or overwhelmed. This leads to a state of **anergy**, where the TST frequently yields a **false-negative** result despite a high bacterial load. **3. High-Yield Clinical Pearls for NEET-PG:** * **Reading the test:** Results are read after **48–72 hours**. Only the **induration** (palpable hardness) is measured, not the erythema. * **False Positive:** Most commonly caused by prior **BCG vaccination** or infection with Nontuberculous Mycobacteria (NTM). * **False Negative (Anergy):** Seen in malnutrition, HIV/AIDS, sarcoidosis, miliary TB, and recent viral infections (e.g., Measles). * **Alternative:** The **IGRA (Interferon-Gamma Release Assay)** is more specific than TST as it does not cross-react with the BCG vaccine.
Question 37: Which human infection spreads through urine?
- A. Leptospira (Correct Answer)
- B. Legionella
- C. Plague
- D. Diphtheria
Explanation: **Explanation:** **Leptospirosis** is a zoonotic disease caused by the spirochete *Leptospira interrogans*. The pathogen colonizes the renal tubules of reservoir animals (primarily rodents, dogs, and livestock) and is shed in their **urine**. Humans acquire the infection through direct contact with infected urine or, more commonly, via water and soil contaminated with it. The bacteria enter the human body through mucous membranes or abraded skin. **Analysis of Options:** * **Leptospira (Correct):** It is the classic example of a "urine-borne" zoonosis. In humans, leptospires can also be found in the urine during the second week of illness (leptospiruria), though human-to-human transmission is rare. * **Legionella:** Spread via **inhalation** of contaminated aerosols from water systems (e.g., AC cooling towers, showers). It is not shed in urine. * **Plague (*Yersinia pestis*):** Primarily transmitted via the **bite of an infected rat flea** (*Xenopsylla cheopis*) or through respiratory droplets (pneumonic plague). * **Diphtheria (*Corynebacterium diphtheriae*):** Transmitted via **respiratory droplets** or direct contact with skin lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Weil’s Disease:** The severe form of Leptospirosis characterized by the triad of **jaundice, renal failure, and hemorrhage.** * **Biphasic Nature:** It presents in two phases: the *Leptospiremic phase* (blood/CSF) followed by the *Immune phase* (urine). * **Culture Media:** Uses specialized media like **EMJH** (Ellinghausen-McCullough-Johnson-Harris) or **Fletcher’s medium**. * **Microscopic Agglutination Test (MAT):** The gold standard for serological diagnosis.
Question 38: Which disease is transmitted by mites?
- A. Endemic typhus
- B. Scrub typhus (Correct Answer)
- C. Epidemic typhus
- D. Trench fever
Explanation: **Explanation:** The correct answer is **Scrub typhus** because it is caused by *Orientia tsutsugamushi* and is transmitted to humans through the bite of the larval stage (chigger) of **trombiculid mites**. These mites serve as both the vector and the natural reservoir (via transovarial transmission). **Analysis of Options:** * **Endemic typhus (Murine typhus):** Caused by *Rickettsia typhi*, it is transmitted by the **rat flea** (*Xenopsylla cheopis*). * **Epidemic typhus:** Caused by *Rickettsia prowazekii*, it is transmitted by the **human body louse** (*Pediculus humanus corporis*). It is known for occurring in crowded conditions like refugee camps. * **Trench fever:** Caused by *Bartonella quintana*, it is also transmitted by the **human body louse**. **High-Yield Clinical Pearls for NEET-PG:** * **The Eschar:** A pathognomonic clinical sign of Scrub typhus is a painless, black, necrotic sore called an **eschar** at the site of the mite bite. * **Weil-Felix Test:** This heterophile agglutination test uses *Proteus* antigens to diagnose rickettsial diseases. Scrub typhus shows a positive reaction with **OX-K** (K for "K"rub typhus), while Epidemic and Endemic typhus react with OX-19 and OX-2. * **Drug of Choice:** **Doxycycline** is the gold standard treatment for all rickettsial infections, including Scrub typhus. * **Geography:** Often associated with "scrub" vegetation (secondary growth of grass/bushes).
Question 39: A patient presents with fever for 3 weeks. On examination, splenomegaly is observed. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?
- A. Cytomegalovirus
- B. Toxoplasmosis
- C. Salmonella (Correct Answer)
- D. Lymphoma virus
Explanation: **Explanation:** The clinical presentation of prolonged fever (3 weeks) and splenomegaly, combined with the isolation of **Gram-negative bacilli** from blood culture, is classic for **Enteric Fever (Typhoid)** caused by *Salmonella Typhi* or *Paratyphi*. 1. **Why Salmonella is correct:** *Salmonella* is a Gram-negative, motile bacillus. During the first and second weeks of infection, it causes bacteremia (positive blood cultures). A known complication of enteric fever is the formation of **splenic abscesses** or focal lesions, which appear as hypoechoic shadows on ultrasonography. Splenomegaly is a hallmark physical finding in the second week of the disease. 2. **Why other options are incorrect:** * **Cytomegalovirus (CMV):** While it causes fever and splenomegaly (mononucleosis-like syndrome), CMV is a virus and would not grow as Gram-negative bacilli on blood culture. * **Toxoplasmosis:** Caused by the protozoan *Toxoplasma gondii*, it typically presents with lymphadenopathy. It does not present as Gram-negative bacilli. * **Lymphoma virus (EBV/HTLV):** These are viruses associated with malignancy or infectious mononucleosis. They do not produce bacterial growth on standard culture media. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Sensitivity:** Blood culture is most sensitive in the **1st week** (90%), while Stool culture is most sensitive in the **3rd week**. * **Widal Test:** Becomes positive only after the **2nd week** (detects antibodies against O and H antigens). * **Pathognomonic sign:** Rose spots on the abdomen (seen in the 2nd week). * **Complications:** Intestinal perforation and hemorrhage typically occur in the **3rd week** due to necrosis of Peyer's patches.
Question 40: A young lady presents with symptoms of Urinary Tract Infection. All of the following findings on a midstream urine sample support the diagnosis of 'Uncomplicated Acute Cystitis', EXCEPT?
- A. Positive Nitrite Test
- B. CFU count < 1000/ml (Correct Answer)
- C. Detection of one bacteria / field on Gram stain
- D. >10WBC/HPF
Explanation: To diagnose **Uncomplicated Acute Cystitis** in a symptomatic female, specific microbiological and microscopic criteria must be met. The diagnosis is primarily clinical, but laboratory findings provide objective support. ### **Explanation of the Correct Answer** **Option B (CFU count < 1000/ml)** is the correct answer because it is **inconsistent** with a diagnosis of UTI. According to the Kass criteria (modified for symptomatic women), a colony-forming unit (CFU) count of **$\geq 10^2$ to $10^3$/ml** of a uropathogen (like *E. coli*) is considered the minimum threshold for significant bacteriuria in symptomatic patients. A count below 1000/ml (specifically $<10^2$) usually suggests contamination rather than an active infection. ### **Analysis of Incorrect Options** * **Option A (Positive Nitrite Test):** This is a highly specific marker for UTI. It indicates the presence of members of the *Enterobacteriaceae* family (e.g., *E. coli*, *Klebsiella*), which possess the enzyme nitrate reductase to convert dietary nitrates into nitrites. * **Option C (One bacteria/field on Gram stain):** In an unspun, midstream urine sample, seeing one or more bacteria per oil-immersion field correlates with a CFU count of $>10^5/ml$, which is diagnostic of significant bacteriuria. * **Option D (>10 WBC/HPF):** This defines **pyuria**. The presence of $>10$ leukocytes per high-power field in centrifuged urine is the most sensitive indicator of an inflammatory response to a urinary tract infection. ### **NEET-PG High-Yield Pearls** * **Gold Standard:** Urine culture remains the gold standard for diagnosis. * **Significant Bacteriuria:** * Symptomatic females: $\geq 10^2$ CFU/ml. * Asymptomatic individuals: $\geq 10^5$ CFU/ml (on two consecutive samples for women). * Suprapubic aspiration: Any growth is significant. * **Sterile Pyuria:** Presence of WBCs but no growth on routine culture. Think *Ureaplasma urealyticum*, *Chlamydia*, or Renal Tuberculosis. * **Most Common Cause:** *Escherichia coli* (Uropathogenic *E. coli* - UPEC).
Practice by Chapter
Respiratory Tract Infections
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Urinary Tract Infections
Practice Questions
Gastrointestinal Infections
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Skin and Soft Tissue Infections
Practice Questions
Central Nervous System Infections
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Bone and Joint Infections
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Cardiovascular Infections
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Sexually Transmitted Infections
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Zoonotic Infections
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Bloodstream Infections and Sepsis
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Fever of Unknown Origin
Practice Questions
Infections in Immunocompromised Host
Practice Questions
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