Infectious Diseases Practice Questions

Q: What is the first sign of tetanus?

Trismus. 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.

Q: An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?

Staphylococcus aureus. 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].

Q: Blood culture is indicated in all, except?

Malaria. **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.

Q: 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?

Chronic malaria. **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.**

Q: Water-lily sign is seen in which of the following conditions?

Complicated hydatid disease. 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**.

Q: The tourniquet test is considered positive if the number of petechiae exceeds which threshold per square inch?

20 petechiae per square inch. **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: $(SBP + DBP) / 2$.

Question 1

What is the first sign of tetanus?

Accepted Answer

Trismus

Answer

Dysphagia

Answer

Neck stiffness

Answer

Back pain

Question 2

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?

Accepted Answer

Cerebellum

Answer

Temporal lobe

Answer

Frontal lobe

Answer

Midbrain

Question 3

Which of the following viral infections is associated with the highest risk of progression to fulminant disease and acute liver failure?

Accepted Answer

Hepatitis D virus (HDV) /Hepatitis B virus (HBV) superinfection

Answer

Hepatitis E virus (HEV) infection

Answer

Hepatitis B virus (HBV) infection

Answer

Hepatitis B virus (HBV)/Hepatitis D virus (HDV) coinfection

Question 4

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?

Accepted Answer

Pneumocystis pneumonia

Answer

Mycobacterial disease

Answer

Kaposi's sarcoma

Answer

Pneumococcal pneumonia

Question 5

An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus gallolyticus

Answer

Pseudomonas aeruginosa

Answer

Streptococcus pyogens

Question 6

Blood culture is indicated in all, except?

Accepted Answer

Malaria

Answer

Enteric fever

Answer

Subacute bacterial endocarditis

Answer

Septicemia

Question 7

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?

Accepted Answer

Chronic malaria

Answer

HIV-AIDS

Answer

Lymphoma

Answer

Disseminated tuberculosis

Question 8

Water-lily sign is seen in which of the following conditions?

Accepted Answer

Complicated hydatid disease

Answer

Hamartoma

Answer

Cavitatory lesion in metastasis of the lung

Answer

Aspergillosis

Question 9

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?

Accepted Answer

diabetes mellitus

Answer

anemia

Answer

exercise

Answer

influenza

Question 10

The tourniquet test is considered positive if the number of petechiae exceeds which threshold per square inch?

Accepted Answer

20 petechiae per square inch

Answer

50 petechiae per square inch

Answer

40 petechiae per square inch

Answer

30 petechiae per square inch

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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