Infectious Diseases — MCQs
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What is the first sign of tetanus?
A 30-year-old male with a past history of otitis media and mastoiditis presented with severe throbbing headache, high-grade fever, nausea, vomiting, confusion, and gait disturbance. On examination, he was febrile, ataxic with nystagmus, and there was no neck rigidity. MRI brain with contrast revealed a brain abscess. Which of the following is the likely site of the brain abscess?
Which of the following viral infections is associated with the highest risk of progression to fulminant disease and acute liver failure?
A patient with HIV is positive, with a viral load of 750,000 copies of HIV RNA/ml and a total CD4 count of 50. This patient is at an increased risk for several infectious diseases. For which of the following diseases does this patient have no added risk compared to an immunocompetent host?
An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?
Blood culture is indicated in all, except?
A young man presents with a history of 6 kg weight loss in 3 months and on examination is found to have generalized lymph node enlargement. Which of the following is the LEAST likely diagnosis?
Water-lily sign is seen in which of the following conditions?
A 68-year-old woman develops new symptoms of burning when voiding. She has no fever, chills, or back discomfort. Her urinalysis reveals numerous white cells and bacteria. Which of the following medical comorbidities is most likely to coexist in this patient?
The tourniquet test is considered positive if the number of petechiae exceeds which threshold per square inch?
Infectious Diseases Indian Medical PG Practice Questions and MCQs
Question 61: What is the first sign of tetanus?
- A. Dysphagia
- B. Neck stiffness
- C. Back pain
- D. Trismus (Correct Answer)
Explanation: Explanation: Tetanus is caused by the neurotoxin tetanospasmin, produced by Clostridium tetani. The toxin acts by blocking the release of inhibitory neurotransmitters (GABA and glycine) from Renshaw cells in the spinal cord, leading to unchecked muscle spasms [1]. Why Trismus is the correct answer: In generalized tetanus, the most common and classic presenting symptom is Trismus (lockjaw). This is due to the masseter muscle being highly sensitive to the toxin, leading to its involuntary contraction. This prevents the patient from opening their mouth fully and is considered the hallmark initial sign [1]. Analysis of Incorrect Options: * A. Dysphagia: While common, difficulty swallowing usually occurs shortly after the onset of trismus as the toxin affects the pharyngeal muscles. * B. Neck stiffness: This is a frequent early sign but typically follows trismus. It contributes to the later development of "Risus Sardonicus" (a characteristic grimace). * C. Back pain: This occurs as the disease progresses to involve the long muscles of the trunk, eventually leading to Opisthotonus (a backward arching of the head, neck, and spine). High-Yield Clinical Pearls for NEET-PG: * Risus Sardonicus: The characteristic facial expression caused by spasms of the facial muscles. * Opisthotonus: The bridge-like arching of the body due to extensor spasm. * Autonomic Instability: The most common cause of death in modern intensive care settings (labile BP, tachycardia, sweating). * Diagnosis: Purely clinical; wound cultures are positive in only ~30% of cases. * Management: Neutralize unbound toxin with Human Tetanus Immune Globulin (HTIG) and use Metronidazole as the preferred antibiotic.
Question 62: A 30-year-old male with a past history of otitis media and mastoiditis presented with severe throbbing headache, high-grade fever, nausea, vomiting, confusion, and gait disturbance. On examination, he was febrile, ataxic with nystagmus, and there was no neck rigidity. MRI brain with contrast revealed a brain abscess. Which of the following is the likely site of the brain abscess?
- A. Temporal lobe
- B. Frontal lobe
- C. Midbrain
- D. Cerebellum (Correct Answer)
Explanation: **Explanation:** The clinical presentation points toward a **Cerebellar Abscess** secondary to contiguous spread from a chronic ear infection [1]. **1. Why Cerebellum is the correct answer:** The patient has a history of **otitis media and mastoiditis**, which are the most common sources of brain abscesses in the posterior fossa. The clinical signs—**ataxia, nystagmus, and gait disturbance**—are classic localizing signs of cerebellar dysfunction. The absence of neck rigidity helps differentiate this from meningitis, while the "throbbing headache" and vomiting indicate increased intracranial pressure [1]. **2. Why other options are incorrect:** * **Temporal Lobe:** While otogenic infections (specifically of the middle ear) frequently spread to the temporal lobe, the presence of **ataxia and nystagmus** specifically localizes the lesion to the cerebellum rather than the temporal lobe (which would present with aphasia or visual field defects). * **Frontal Lobe:** Frontal lobe abscesses usually arise from **paranasal sinusitis** (frontal or ethmoid) or dental infections. They typically present with personality changes, hemiparesis, or seizures. * **Midbrain:** Brainstem abscesses are extremely rare and would present with cranial nerve palsies and long-tract signs (hemiplegia/sensory loss) rather than isolated cerebellar signs. **Clinical Pearls for NEET-PG:** * **Source vs. Location:** * Otitis Media/Mastoiditis $→$ Temporal lobe or Cerebellum. * Sinusitis $→$ Frontal lobe. * Hematogenous spread (e.g., Endocarditis) $→$ Multiple abscesses, often in the distribution of the Middle Cerebral Artery (MCA). * **Triad of Brain Abscess:** Headache, fever, and focal neurological deficit (present in <50% of cases). * **Imaging:** Contrast-enhanced MRI is the gold standard, typically showing a **ring-enhancing lesion**. [1] * **Contraindication:** Lumbar puncture is contraindicated if a brain abscess is suspected due to the risk of herniation.
Question 63: Which of the following viral infections is associated with the highest risk of progression to fulminant disease and acute liver failure?
- A. Hepatitis E virus (HEV) infection
- B. Hepatitis B virus (HBV) infection
- C. Hepatitis B virus (HBV)/Hepatitis D virus (HDV) coinfection
- D. Hepatitis D virus (HDV) /Hepatitis B virus (HBV) superinfection (Correct Answer)
Explanation: The risk of fulminant hepatic failure in viral hepatitis depends on the specific virus and the context of the infection [1]. **Why Option D is Correct:** **HDV Superinfection** occurs when a patient who is already a chronic carrier of Hepatitis B (HBsAg positive) becomes infected with HDV. This scenario carries the **highest risk (up to 20%)** of progressing to fulminant disease and acute liver failure. Because the HBV replication machinery is already established, HDV can replicate aggressively, leading to severe, rapid hepatocellular necrosis. It also frequently leads to chronic HDV infection and rapid progression to cirrhosis. **Analysis of Incorrect Options:** * **Option A (HEV):** While HEV is a common cause of acute liver failure in developing countries, its risk is generally low in the general population. It is only "fulminant" in a specific high-yield context: **pregnant women** (mortality rate ~20%). * **Option B (HBV):** Most acute HBV infections in adults are self-limiting; less than 1% progress to fulminant hepatic failure. * **Option C (HBV/HDV Coinfection):** This occurs when a person acquires both viruses simultaneously. While it often causes a more severe acute illness than HBV alone, the risk of fulminance is lower than in superinfection (approx. 2–5%), and it rarely leads to chronic infection. **High-Yield NEET-PG Pearls:** * **Coinfection:** Simultaneous HBV + HDV; usually self-limiting; HBsAg and IgM anti-HBc are both positive. * **Superinfection:** HDV on top of Chronic HBV; high risk of fulminant failure; IgM anti-HBc is negative (since HBV is chronic), but HBsAg and anti-HDV are positive. * **HEV:** Highest mortality in the **3rd trimester** of pregnancy. * **HBV:** The most common cause of chronic liver disease worldwide, but HDV superinfection is the most "explosive" clinical event [1].
Question 64: A patient with HIV is positive, with a viral load of 750,000 copies of HIV RNA/ml and a total CD4 count of 50. This patient is at an increased risk for several infectious diseases. For which of the following diseases does this patient have no added risk compared to an immunocompetent host?
- A. Pneumocystis pneumonia (Correct Answer)
- B. Mycobacterial disease
- C. Kaposi's sarcoma
- D. Pneumococcal pneumonia
Explanation: The correct answer is **A. Pneumocystis pneumonia**. ### **Explanation** The question asks for which disease the patient has **no added risk** compared to an immunocompetent host. This is a conceptual trap. * **Pneumocystis jirovecii pneumonia (PCP)** is an **obligate opportunistic infection**. It almost exclusively occurs in individuals with significant immunosuppression (CD4 < 200 cells/µL) [1], [2]. * An **immunocompetent host** has virtually **zero risk** of developing clinical PCP. Therefore, while the HIV patient is at high risk, the "added risk" comparison is technically invalid because the baseline risk in the general population is non-existent. * *Note:* In many standard medical examinations, this question highlights that while HIV patients are susceptible to many infections, some (like Pneumococcus) affect both healthy and ill people, whereas others (like PCP) are unique to the immunocompromised. ### **Why the other options are wrong:** * **B. Mycobacterial disease:** HIV patients have a significantly higher risk of both *M. tuberculosis* (reactivation and primary) and *M. avium* complex (MAC) compared to the general population [1], [2]. * **C. Kaposi's sarcoma:** This is an AIDS-defining illness caused by HHV-8. It is extremely rare in immunocompetent individuals (except in specific endemic or classic forms). * **D. Pneumococcal pneumonia:** *Streptococcus pneumoniae* is the most common cause of bacterial pneumonia in HIV patients [2]. The risk of invasive pneumococcal disease is **10 to 100 times higher** in HIV-positive individuals than in immunocompetent hosts. ### **High-Yield Clinical Pearls for NEET-PG:** 1. **CD4 < 200:** Threshold for PCP prophylaxis (Trimethoprim-Sulfamethoxazole) [1]. 2. **CD4 < 50:** High risk for CMV retinitis and *Mycobacterium avium* complex (MAC) [1]. 3. **Most common pneumonia in HIV:** Still *Streptococcus pneumoniae*, though PCP is the most common opportunistic respiratory infection [2]. 4. **PCP Diagnosis:** Silver stain (Gomori) showing "crushed ping-pong ball" cysts; elevated LDH is a common non-specific marker [2].
Question 65: An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?
- A. Staphylococcus aureus (Correct Answer)
- B. Streptococcus gallolyticus
- C. Pseudomonas aeruginosa
- D. Streptococcus pyogens
Explanation: Explanation: **1. Why Staphylococcus aureus is correct:** *Staphylococcus aureus* is the most common cause of infective endocarditis (IE) in **Intravenous Drug Users (IVDUs)** [1]. In these patients, the infection typically involves the **right side of the heart**, specifically the **tricuspid valve** (approx. 60-70% of cases). The organism enters the bloodstream through skin flora during non-sterile injections and seeds the tricuspid valve [1], [3]. *S. aureus* is highly virulent, often leading to acute IE with large, friable vegetations and a high risk of septic pulmonary emboli [1]. **2. Why the other options are incorrect:** * **Streptococcus gallolyticus (formerly S. bovis):** This is strongly associated with **colorectal cancer** or polyps [1]. If isolated in IE, the next mandatory step is a colonoscopy. * **Pseudomonas aeruginosa:** While it is a known cause of IE in IVDUs (especially in certain geographic locations like Detroit or Chicago), it is significantly less common than *S. aureus*. * **Streptococcus pyogenes:** This is a common cause of skin infections and rheumatic fever but is a rare cause of acute infective endocarditis compared to *S. aureus* [1]. **3. NEET-PG High-Yield Pearls:** * **Most common valve involved in IVDU:** Tricuspid Valve (Right-sided IE). * **Most common valve involved overall (Non-IVDU):** Mitral Valve. * **Most common organism in Subacute IE:** *Streptococcus viridans* [1]. * **Most common organism in Prosthetic Valve IE (<1 year):** *Staphylococcus epidermidis* [3]. * **Clinical Sign:** IVDU patients with tricuspid IE often present with "septic pulmonary infarcts" (nodular opacities on Chest X-ray) rather than peripheral emboli like Janeway lesions [2].
Question 66: Blood culture is indicated in all, except?
- A. Enteric fever
- B. Subacute bacterial endocarditis
- C. Septicemia
- D. Malaria (Correct Answer)
Explanation: **Explanation:** The correct answer is **Malaria (Option D)**. **Why Malaria is the correct answer:** Malaria is caused by protozoan parasites of the genus *Plasmodium*. These parasites reside within erythrocytes (RBCs) and are diagnosed via **microscopic examination** of peripheral blood smears (thick and thin smears) or **Rapid Diagnostic Tests (RDTs)** that detect parasite antigens (e.g., HRP-2, LDH). Since it is a parasitic infection [3] and not a bacterial one, standard aerobic or anaerobic blood culture media will not support its growth. **Why the other options are incorrect:** * **Enteric Fever (Option A):** Blood culture is the gold standard for diagnosis in the **first week** of illness. *Salmonella Typhi* is a bacterium that can be isolated from blood during the bacteremic phase. * **Subacute Bacterial Endocarditis (Option B):** Blood cultures are the most critical diagnostic tool for SBE (part of the **Duke Criteria**). Continuous bacteremia occurs in endocarditis, and multiple sets of cultures are required to identify the causative organism (e.g., *Viridans streptococci*) [1]. * **Septicemia (Option C):** By definition, septicemia involves the presence and multiplication of bacteria in the blood. Blood culture is mandatory to identify the pathogen and determine antibiotic sensitivity [2]. **Clinical Pearls for NEET-PG:** * **Enteric Fever:** Remember the mnemonic **BASU** for culture positivity: **B**lood (1st week), **A**gglutination/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). **Bone marrow culture** is the most sensitive overall. * **Blood Culture Volume:** In adults, 10–20 ml of blood per bottle is recommended to increase the yield. * **Malaria Diagnosis:** The **thick smear** is used for screening (higher sensitivity), while the **thin smear** is used for species identification and calculating the parasite index.
Question 67: A young man presents with a history of 6 kg weight loss in 3 months and on examination is found to have generalized lymph node enlargement. Which of the following is the LEAST likely diagnosis?
- A. HIV-AIDS
- B. Lymphoma
- C. Chronic malaria (Correct Answer)
- D. Disseminated tuberculosis
Explanation: **Explanation:** The clinical presentation of significant weight loss (6 kg in 3 months) and **generalized lymphadenopathy** (enlargement of two or more non-contiguous lymph node stations) suggests a systemic chronic infection or malignancy [1]. **Why Chronic Malaria is the correct (least likely) answer:** Malaria typically presents with fever, anemia, and **splenomegaly**. While "Tropical Splenomegaly Syndrome" (Hyperreactive Malarial Splenomegaly) can occur in chronic cases, it does **not** typically cause generalized lymphadenopathy or profound weight loss. Lymph node enlargement is not a characteristic feature of the *Plasmodium* life cycle in humans. **Analysis of Incorrect Options:** * **HIV-AIDS:** Persistent Generalized Lymphadenopathy (PGL) is a hallmark of early symptomatic HIV [3]. Significant weight loss (Wasting Syndrome) is a defining feature of progression to AIDS [3]. * **Lymphoma:** Both Hodgkin and Non-Hodgkin Lymphomas are classic causes of generalized lymphadenopathy [2]. "B-symptoms," including significant unexplained weight loss (>10% in 6 months), are key diagnostic criteria [2]. * **Disseminated Tuberculosis:** In endemic regions like India, TB is a leading cause of generalized lymphadenopathy (miliary or disseminated TB) [1]. It is almost always accompanied by constitutional symptoms like weight loss and night sweats [1]. **NEET-PG Clinical Pearls:** * **Definition:** Generalized lymphadenopathy involves $\ge$ 2 non-contiguous node stations. * **Mnemonic for Generalized Lymphadenopathy (MIAMI):** **M**alignancy, **I**nfectious (HIV, EBV, TB), **A**utoimmune (SLE, RA), **M**iscellaneous (Sarcoidosis), **I**atrogenic (Phenytoin) [2]. * **Weight Loss:** In NEET-PG questions, "Weight loss + Lymphadenopathy" should first trigger thoughts of **TB, HIV, or Malignancy.**
Question 68: Water-lily sign is seen in which of the following conditions?
- A. Hamartoma
- B. Complicated hydatid disease (Correct Answer)
- C. Cavitatory lesion in metastasis of the lung
- D. Aspergillosis
Explanation: The **Water-lily sign** (also known as the Camelot sign) is a classic radiological finding pathognomonic for a **complicated hydatid cyst**, specifically when it occurs in the lungs. [1] **1. Why the correct answer is right:** Hydatid disease is caused by the parasite *Echinococcus granulosus*. [1] A "complicated" cyst refers to one that has ruptured. When the endocyst (the inner germinal layer) ruptures, it collapses and floats on top of the remaining fluid within the ectocyst (the outer pericyst). [1] On a chest X-ray or CT scan, these detached, undulating membranes floating on the fluid level resemble the leaves of a water lily. **2. Why the incorrect options are wrong:** * **Hamartoma:** Typically presents as a well-defined pulmonary nodule with "popcorn calcification," not floating membranes. * **Cavitatory metastasis:** While some metastases (like squamous cell carcinoma) can cavitate, they usually present with thick, irregular walls and lack the specific floating membrane sign. * **Aspergillosis:** An Aspergilloma (fungal ball) produces the **Monod sign** or **Air-crescent sign**, where a solid mass of hyphae sits at the bottom of a pre-existing cavity with a thin rim of air above it. [2] **3. NEET-PG High-Yield Pearls:** * **Causative Agent:** *Echinococcus granulosus* (Dog tapeworm). [1] * **Most common organ involved:** Liver (presents with "Cyst within a cyst" or "Honeycomb appearance"). * **Second most common organ:** Lungs. * **Other Signs:** * **Whirl sign:** On CT, representing the detached membranes. * **Casoni Test:** Immediate hypersensitivity skin test (now largely replaced by Serology/ELISA). * **Management:** PAIR (Puncture, Aspiration, Injection, Re-aspiration) is used for liver cysts, but **surgery** is preferred for lung cysts to avoid anaphylaxis and bronchial rupture. Medical management involves **Albendazole**.
Question 69: A 68-year-old woman develops new symptoms of burning when voiding. She has no fever, chills, or back discomfort. Her urinalysis reveals numerous white cells and bacteria. Which of the following medical comorbidities is most likely to coexist in this patient?
- A. anemia
- B. exercise
- C. diabetes mellitus (Correct Answer)
- D. influenza
Explanation: The patient presents with classic symptoms of an uncomplicated **Urinary Tract Infection (UTI)**: dysuria (burning on voiding) and pyuria (white cells in urine) without systemic signs like fever or flank pain (which would suggest pyelonephritis) [1]. **Why Diabetes Mellitus is the correct answer:** Diabetes mellitus is the most significant risk factor among the choices for developing UTIs. Several mechanisms contribute to this: 1. **Glucosuria:** High glucose levels in the urine provide an ideal culture medium for bacterial growth. 2. **Immune Dysfunction:** Hyperglycemia impairs neutrophil function (chemotaxis and adherence) and cytokine release. 3. **Autonomic Neuropathy:** Long-standing diabetes can lead to a "neurogenic bladder," causing incomplete emptying (urinary stasis), which facilitates bacterial colonization. **Why other options are incorrect:** * **Anemia:** While chronic disease can weaken the body, there is no direct pathophysiological link between anemia and an increased incidence of UTIs. * **Exercise:** Physical activity does not predispose an individual to UTIs; in fact, it generally promotes overall immune health. * **Influenza:** This is a respiratory viral infection. While it causes systemic illness, it is not a recognized risk factor for bacterial cystitis. **NEET-PG High-Yield Pearls:** * **Most common organism:** *E. coli* remains the most common cause of UTI in both diabetic and non-diabetic patients. * **Emphysematous Cystitis/Pyelonephritis:** These are severe, gas-forming infections of the urinary tract seen almost exclusively in patients with **Diabetes Mellitus**. * **Asymptomatic Bacteriuria:** In diabetic patients, asymptomatic bacteriuria is common, but current guidelines (IDSA) recommend treatment **only** if the patient is pregnant or undergoing urological procedures [2]. * **Klebsiella and Enterococcus:** These organisms are more frequently isolated in diabetic patients compared to the general population.
Question 70: The tourniquet test is considered positive if the number of petechiae exceeds which threshold per square inch?
- A. 50 petechiae per square inch
- B. 40 petechiae per square inch
- C. 30 petechiae per square inch
- D. 20 petechiae per square inch (Correct Answer)
Explanation: **Explanation:** The **Tourniquet Test** (also known as the Hess test) is a clinical diagnostic tool used to assess capillary fragility and is a key component of the WHO case definition for **Dengue Hemorrhagic Fever (DHF)**. **Why Option D is correct:** The test is performed by inflating a blood pressure cuff to a point midway between the systolic and diastolic blood pressure for 5 minutes. After deflating the cuff and waiting 2 minutes for skin color to return to normal, the petechiae are counted within a 1-square-inch (2.5 cm x 2.5 cm) area. A result is considered **positive** if there are **20 or more petechiae per square inch**. This indicates increased capillary permeability and fragility, often seen in the setting of thrombocytopenia or vascular dysfunction associated with Dengue. **Why other options are incorrect:** * **Options A, B, and C (50, 40, 30 petechiae):** These thresholds are significantly higher than the standardized WHO criteria. Using these values would result in a high false-negative rate, failing to identify patients at risk for plasma leakage and hemorrhagic complications. **NEET-PG High-Yield Pearls:** * **Dengue Classification:** The tourniquet test is a marker of "Dengue with warning signs" according to the 2009 WHO classification. * **Sensitivity:** While specific, the test may be negative during the early febrile phase or in patients with profound shock. * **Differential Diagnosis:** Besides Dengue, a positive test can be seen in ITP, Vitamin C deficiency (Scurvy), and Scarlet Fever. * **Calculation Tip:** The pressure used is the **Mean Arterial Pressure (MAP)** or simply the midpoint: $(SBP + DBP) / 2$.
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Principles of Antimicrobial Therapy
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Fever of Unknown Origin
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HIV/AIDS and Related Infections
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Tuberculosis and Mycobacterial Diseases
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Tropical and Parasitic Infections
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Viral Infections (Hepatitis, Herpes, etc.)
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Healthcare-Associated Infections
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Fungal Infections
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Sepsis and Septic Shock
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Infection in Immunocompromised Hosts
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Emerging and Re-emerging Infections
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Antimicrobial Resistance
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Vaccination Principles
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