Infectious Diseases Practice Questions

Q: Saddleback/biphasic fever is seen in:

Dengue. **Explanation:** **Dengue fever** is the classic cause of a **saddleback (biphasic) fever** pattern. This characteristic profile consists of an initial high-grade fever lasting 3–4 days, followed by a brief period of remission (defervescence) lasting 1–2 days, and a subsequent second rise in temperature for another 2–3 days. The second phase often coincides with the appearance of a generalized morbilliform rash [1]. **Analysis of Incorrect Options:** * **Typhoid (Enteric Fever):** Characterized by a **"Step-ladder"** pattern of fever, where the temperature rises gradually over the first week. * **Brucellosis:** Classically associated with an **"Undulant"** fever pattern, featuring waves of temperature that rise and fall over weeks to months. * **Trench Fever:** Caused by *Bartonella quintana*, it typically presents as a **"Quintan"** fever (recurring every 5 days). **High-Yield Clinical Pearls for NEET-PG:** 1. **Other causes of Saddleback fever:** Yellow fever, Colorado tick fever, Rift Valley fever, and occasionally Poliomyelitis or Leptospirosis [3]. 2. **Dengue Triad:** High fever, retro-orbital pain, and severe backache/myalgia (hence the name "Break-bone fever") [2]. 3. **Herman’s Sign:** Seen in Dengue; it is a late-onset maculopapular rash described as "islands of white in a sea of red." 4. **Tourniquet Test:** A positive test (≥10 petechiae per square inch) is a clinical indicator of capillary fragility in Dengue [1].

Q: Persistent marrow aplasia is caused by which hepatitis virus?

None of the above. **Explanation:** The correct answer is **None of the above** because persistent marrow aplasia (Aplastic Anemia) is most commonly associated with **Non-A, Non-B, Non-C, Non-E, and Non-G hepatitis viruses**. **1. Understanding the Concept:** Post-hepatitic aplastic anemia is a distinct clinical syndrome where bone marrow failure follows an episode of acute hepatitis. In approximately 90% of these cases, the known hepatotropic viruses (HAV, HBV, HCV, HEV) are **not** the causative agents. Instead, it is attributed to unidentified "Non-A-E" viruses or an aberrant T-cell mediated immune response triggered by an unknown viral infection that cross-reacts with hematopoietic stem cells. **2. Analysis of Options:** * **Hepatitis A (HAV):** While rare cases of transient marrow suppression have been reported, it does not typically cause persistent aplastic anemia. * **Hepatitis B (HBV) & Hepatitis C (HCV):** These are primarily associated with chronic liver disease, cirrhosis, and hepatocellular carcinoma. While they can cause extrahepatic manifestations (like polyarteritis nodosa for HBV or cryoglobulinemia for HCV), they are not the standard causes of post-hepatitic marrow aplasia. **3. NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Post-hepatitic aplastic anemia typically occurs 2–3 months after the acute hepatitis episode. * **Prognosis:** It is often severe and can be fatal if not treated with bone marrow transplantation or immunosuppressive therapy (Antithymocyte globulin + Cyclosporine). * **Most Common Cause:** Idiopathic (60-70%), followed by drugs/toxins and post-hepatitic (Non-A-E) syndromes. * **Hepatitis E (HEV):** In pregnant women, HEV is associated with high mortality due to Fulminant Hepatic Failure, not marrow aplasia.

Q: A patient with prosthetic valve replacement develops endocarditis 8 months later. Which organism is most likely responsible?

Staphylococcus epidermidis. **Explanation:** Prosthetic Valve Endocarditis (PVE) is classified based on the timing of symptom onset after surgery, which dictates the most likely causative organism: 1. **Early PVE (<12 months):** The correct answer is **Staphylococcus epidermidis** (Coagulase-negative Staphylococcus) [1]. During surgery, the prosthetic material is easily colonized by skin flora [1]. *S. epidermidis* produces a **biofilm** (extracellular polysaccharide matrix) that protects it from antibiotics and host immune responses, making it the most common cause of PVE within the first year. 2. **Late PVE (>12 months):** The microbiology resembles native valve endocarditis, where **Streptococcus viridans** becomes the most common cause due to transient bacteremia from dental or mucosal sources. **Analysis of Incorrect Options:** * **Staphylococcus aureus (A):** While a common cause of acute endocarditis and early PVE, it is generally less frequent than *S. epidermidis* in the subacute presentation of early prosthetic infections. * **Streptococcus viridans (B):** This is the leading cause of **Late PVE** (>1 year post-surgery) and community-acquired native valve endocarditis. * **HACEK group (D):** These are fastidious gram-negative organisms (e.g., *Haemophilus, Aggregatibacter*) that typically cause **culture-negative endocarditis** and are rarely the primary cause of early PVE. **High-Yield Pearls for NEET-PG:** * **Most common cause of NVE (Native Valve):** *Streptococcus viridans*. * **Most common cause in IV drug users:** *Staphylococcus aureus* (often affecting the **Tricuspid valve**). * **Duke’s Criteria:** The gold standard for diagnosis (requires 2 major, 1 major + 3 minor, or 5 minor criteria) [2]. * **Biofilm formation:** The key virulence factor for *S. epidermidis* in prosthetic device infections [1].

Q: Cerebrospinal fluid (CSF) adenosine deaminase levels are significantly higher in which of the following conditions?

Tuberculous meningitis. **Explanation:** **Adenosine Deaminase (ADA)** is an enzyme involved in purine metabolism, primarily produced by T-lymphocytes during their activation and proliferation. Its measurement in body fluids serves as a surrogate marker for cell-mediated immunity. **1. Why Tuberculous Meningitis (TBM) is correct:** In TBM, the body mounts a robust cell-mediated immune response [1]. The infiltration and activation of T-lymphocytes in the subarachnoid space lead to significantly elevated levels of ADA in the CSF [2]. A cut-off value of **>10 U/L** is highly suggestive of TBM, with high sensitivity and specificity, making it a crucial rapid diagnostic tool while waiting for culture results. **2. Why other options are incorrect:** * **Bacterial Meningitis:** While ADA can be slightly elevated due to neutrophils, the levels are typically much lower than in TBM [3]. The primary diagnostic markers here are low glucose, high protein, and a predominant neutrophilic pleocytosis. * **Viral Meningitis:** This condition usually presents with normal or only minimally elevated ADA levels because the lymphocytic response is not as intense as the granulomatous inflammation seen in TB [3]. * **Syphilitic Meningitis:** Although it causes a lymphocytic pleocytosis, it does not typically trigger the specific T-cell activation pathway that results in high ADA levels. **Clinical Pearls for NEET-PG:** * **Gold Standard for TBM:** CSF Culture (BACTEC/MGIT) or GeneXpert (NAAT). * **CSF Profile in TBM:** Fibrin web/cobweb formation, high protein, low glucose, and lymphocytic pleocytosis [3]. * **ADA in other fluids:** Elevated ADA is also highly diagnostic in **Pleural effusion** and **Peritoneal fluid** (Ascites) for Tuberculosis. * **False Positives:** CSF ADA can also be elevated in CNS Lymphoma and Neurobrucellosis.

Q: What is true about Rabies?

Bats transmit the disease in some parts of the world.. **Explanation:** **1. Why Option A is correct:** Rabies is a zoonotic viral disease caused by Lyssaviruses. While domestic dogs are the primary source of human rabies deaths globally (especially in Asia and Africa), **bats** are the most common source of rabies in the Americas and parts of Europe. In these regions, bat-mediated rabies is a significant public health concern, making this statement factually accurate. **2. Why the other options are incorrect:** * **Option B:** While rabies involves the brainstem, the characteristic pathology is **Limbic Encephalitis**. The virus primarily affects the hippocampus (Ammon’s horn) and the cerebellum (Purkinje cells), rather than being isolated to the brainstem. * **Option C:** Negri bodies (pathognomonic eosinophilic cytoplasmic inclusions) are most commonly found in the **Hippocampus (Ammon’s horn)** and the **Purkinje cells of the cerebellum**. They are not typically associated with the anterior pituitary. * **Option D:** **Paraesthesia** (tingling, itching, or pain) at the site of the bite is actually the **most important early diagnostic sign** of rabies, occurring in 50-80% of patients during the prodromal phase. **Clinical Pearls for NEET-PG:** * **Incubation Period:** Usually 1–3 months (highly variable based on the distance of the bite from the CNS). * **Hydrophobia:** Pathognomonic sign caused by forceful spasms of the diaphragm and accessory respiratory muscles when attempting to swallow. * **Diagnosis:** Intra-vitam diagnosis is made via **Skin biopsy** (from the nape of the neck) for viral antigen or **PCR** of saliva/CSF. * **Post-Exposure Prophylaxis (PEP):** Includes wound washing, Rabies Vaccine (Days 0, 3, 7, 14, 28), and Rabies Immunoglobulin (RIG) for Category III bites [1].

Q: A 65-year-old woman undergoes a kidney transplant for CKD. Six weeks after the transplant, she develops CMV infection characterized by fever, malaise, and myalgias. Which of the following is a potential sequela of CMV infection?

GI ulcers and hemorrhage. **Explanation:** Cytomegalovirus (CMV) is the most common opportunistic viral infection in solid organ transplant recipients, typically manifesting within the first 1–6 months post-transplant due to peak immunosuppression. **Why Option D is Correct:** CMV has a particular tropism for vascular endothelial cells. In the gastrointestinal tract, it causes **endothelial damage and vasculitis**, leading to mucosal ischemia and subsequent **ulceration**. These ulcers are typically "punched-out" and can occur anywhere from the esophagus to the colon, leading to symptoms of odynophagia, abdominal pain, and significant **GI hemorrhage**. In renal transplant patients, CMV is also a major cause of graft dysfunction and "CMV syndrome" (fever and cytopenias). **Why Other Options are Incorrect:** * **Options A & B:** Intra-abdominal abscesses and pyelonephritis are typically **bacterial** complications (e.g., *E. coli*, *Klebsiella*). While transplant patients are at risk for these, they are not specific sequelae of CMV infection. * **Option C:** While CMV can rarely cause acalculous cholecystitis in advanced AIDS patients, it is not a classic or common sequela in the post-transplant period compared to GI involvement. **High-Yield Clinical Pearls for NEET-PG:** * **Histology:** Look for **"Owl’s eye" inclusion bodies** (large intranuclear inclusions with a clear halo). * **Diagnosis:** **pp65 antigenemia** assay or **PCR** for CMV DNA are the preferred diagnostic methods. * **Treatment:** **Ganciclovir** is the drug of choice; Valganciclovir is used for prophylaxis. * **Congenital CMV:** Most common cause of non-syndromic sensorineural hearing loss and periventricular calcifications.

Question 1

Saddleback/biphasic fever is seen in:

Accepted Answer

Dengue

Answer

Typhoid

Answer

Brucellosis

Answer

Trench fever

Question 2

Which parameter is best for monitoring HIV disease progression?

Accepted Answer

Peripheral blood CD4+ T-cell counts with plasma levels of HIV RNA

Answer

Peripheral blood CD4+ T-cell counts

Answer

Plasma levels of HIV RNA

Answer

Peripheral blood CD4+ T-cell counts or plasma levels of HIV RNA

Question 3

Persistent marrow aplasia is caused by which hepatitis virus?

Accepted Answer

None of the above

Answer

Hepatitis A virus (HAV)

Answer

Hepatitis B virus (HBV)

Answer

Hepatitis C virus (HCV)

Question 4

A patient with prosthetic valve replacement develops endocarditis 8 months later. Which organism is most likely responsible?

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Streptococcus viridans

Answer

HACEK group

Question 5

Which of the following statements regarding brain abscess is FALSE?

Accepted Answer

Abscesses that develop as a result of direct spread of infection from the sinuses and dental infections are usually located in the frontal lobe.

Answer

The most common source of infection is direct spread from a contiguous cranial site of infection, such as paranasal sinusitis, otitis media, mastoiditis, or dental infection.

Answer

Hematogenous abscesses are often multiple, and multiple abscesses often (50%) have a hematogenous origin.

Answer

Hematogenous abscesses show a predilection for the territory of the middle cerebral artery.

Question 6

Which of the following is the LEAST common immunologic manifestation of HIV infection?

Accepted Answer

Anaphylactic reactions to drugs

Answer

Cutaneous reactions to drugs

Answer

Anticardiolipin antibodies

Answer

Oligoarticular arthritis

Question 7

Cerebrospinal fluid (CSF) adenosine deaminase levels are significantly higher in which of the following conditions?

Accepted Answer

Tuberculous meningitis

Answer

Bacterial meningitis

Answer

Viral meningitis

Answer

Syphilitic meningitis

Question 8

A patient with diarrhea has a fecal smear that is negative for leukocytes. The patient's diarrhea is most likely caused by

Accepted Answer

Enterotoxigenic Escherichia coli

Answer

Campylobacter enteritis

Answer

Shigellosis

Answer

Typhoid fever

Question 9

What is true about Rabies?

Accepted Answer

Bats transmit the disease in some parts of the world.

Answer

Brainstem encephalitis is characteristic.

Answer

Negri bodies are commonly found in the thalamus and anterior pituitary.

Answer

Paraesthesia is not seen.

Question 10

A 65-year-old woman undergoes a kidney transplant for CKD. Six weeks after the transplant, she develops CMV infection characterized by fever, malaise, and myalgias. Which of the following is a potential sequela of CMV infection?

Accepted Answer

GI ulcers and hemorrhage

Answer

Intra-abdominal abscess

Answer

Pyelonephritis

Answer

Cholecystitis

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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