Q.Give an example of a rod shaped virus.

The key here is understanding what a prion is — not a virus, not a bacterium, but a misfolded protein that can induce normal proteins in the brain to also misfold, leading to fatal brain damage. This is a completely different mechanism from infections caused by pathogens like bacteria or viruses.

Mad cow disease, formally called bovine spongiform encephalopathy (BSE), is a prion disease in cattle. When humans eat beef contaminated with the BSE prion, the prion can cross species and cause a similar disease in humans. That human disease is called variant Creutzfeldt–Jakob disease (vCJD). It is distinct from the classic Creutzfeldt–Jakob disease, which occurs spontaneously, but both are prion diseases.

Now let’s eliminate the other options:

1. Sleeping sickness is caused by the protozoan parasite Trypanosoma brucei, transmitted by the tsetse fly. No prions involved. …

The question tests your knowledge of protozoan diseases and their specific causative agents. Leishmania is a genus of parasitic protozoa transmitted by sandflies, but different species cause different clinical forms. Leishmania donovani is the classic agent of visceral leishmaniasis, also known as Kala-azar — a systemic infection affecting the spleen, liver, and bone marrow. The other options are caused by different species or entirely different pathogens.

1. Identify the pathogen and its disease

   Leishmania donovani is a flagellate protozoan. It is transmitted by the bite of infected female Phlebotomus sandflies. Once inside the human body, it invades macrophages and multiplies, leading to a chronic systemic illness. The hallmark symptoms are prolonged fever, weight loss, anaemia, and massive enlargement of the spleen and liver. This clinical picture is called Kala-azar (meaning "black fever" in Hindi, referring to the darkening of skin sometimes seen).

2. Eliminate the other options

   • (B) Delhi boils — This is a form of cutaneous leishmaniasis, caused by Leishmania tropica (not donovani). It produces skin ulcers, not systemic disease. …

Understanding the Malaria Transmission Cycle

To evaluate these statements, we need to understand how malaria spreads and why mosquito control is central to prevention strategies.

The biological context: Malaria is caused by Plasmodium parasites (several species including P. falciparum, P. vivax, etc.). These parasites cannot jump directly from human to human—they require a vector to complete their life cycle.

The vector: Female Anopheles mosquitoes serve as the obligate vector. When a female mosquito bites an infected person, it ingests Plasmodium gametocytes. The parasite undergoes sexual reproduction in the mosquito's gut, eventually producing sporozoites that migrate to the salivary glands. When this mosquito bites another person, it injects (inoculates) sporozoites along with anticoagulant saliva.

Evaluating Each Statement

Let's examine the assertion and reason systematically:

1. Assertion (A): "To control malaria, mosquitoes are to be eradicated"

   This is true. Vector control through mosquito eradication (or reduction) is one of the most effective malaria control strategies. Historical successes include DDT spraying campaigns, bed net distribution, and elimination of breeding sites. Breaking the transmission cycle by removing the vector prevents new infections.

2. Reason (R): "Mode of infection of Plasmodium is by inoculation (by female Anopheles)"

   This is true. The term "inoculation" accurately describes the injection of sporozoites into human blood during a mosquito bite. Only female mosquitoes bite (they need blood protein for egg development), and only Anopheles species transmit human malaria effectively. …

The key here is to understand what each technique actually does — not just its name, but its purpose. The question specifically mentions "antibodies synthesised by host against the pathogen." That means we are looking for a method that identifies the presence of specific antibodies in a sample (like blood or serum). This is a core idea in immunology and diagnostics.

Let’s go through the options one by one.

1. Polymerase Chain Reaction (PCR) – This amplifies DNA, not antibodies. It detects the genetic material of the pathogen itself, not the host’s immune response. So it’s out.

2. ELISA (Enzyme-Linked Immunosorbent Assay) – This is exactly what we need. ELISA works by using an enzyme-linked antibody to detect a specific antigen or antibody. In the case of detecting host antibodies, the test is set up with the pathogen’s antigen fixed to a plate. If the host’s antibodies are present, they bind to the antigen. Then a second, enzyme-linked antibody (that binds to host antibodies) is added, and a colour change reveals the result. This is a standard diagnostic tool for infections like HIV, hepatitis, and many others.

3. DNA fingerprinting – This compares patterns in DNA (like VNTRs) to identify individuals. It has nothing to do with detecting antibodies. It’s used in forensics and paternity testing, not in immune response detection. …

The question asks where merozoites of Plasmodium (the malaria parasite) transform into gametocytes — the sexual stage that can infect mosquitoes. This is a classic point of confusion because many students think gametocytes form in the blood or liver, but recent research has clarified the actual site.

Why this approach works: Understanding the parasite’s life cycle is essential. After merozoites invade red blood cells, most develop into asexual forms (causing symptoms), but a small fraction commit to sexual development. The bone marrow provides a unique microenvironment — with lower oxygen tension and specific host factors — that triggers this switch. The liver is for initial schizogony, the kidney and lymph are not primary sites for Plasmodium development.

Step-by-step reasoning:

1. Recall the Plasmodium life cycle basics.

   • Sporozoites from a mosquito bite first infect liver cells (hepatocytes), where they multiply into merozoites.
   • Merozoites then invade red blood cells (erythrocytes) in the bloodstream.
   • Inside erythrocytes, most merozoites undergo asexual replication (schizogony), causing the cyclical fevers of malaria.

2. Identify the gametocyte formation trigger.

   • A small subset of merozoites, instead of continuing asexual replication, differentiate into gametocytes (male and female).
   • This differentiation is not random — it is influenced by environmental cues, particularly the microenvironment of the bone marrow.

3. Pinpoint the anatomical site.

   • Studies (e.g., using Plasmodium falciparum models) show that immature gametocytes are found sequestered in the bone marrow and spleen, not in circulating blood.
   • The bone marrow’s low oxygen levels and specific cytokines promote the development of gametocytes from merozoite-infected erythrocytes. …

Let’s examine each statement one by one, grounding every judgment in clear biological principles.

1. Statement A: “No virus contains both DNA and RNA.”

   This is a fundamental rule of virology. A virus particle (virion) has either DNA or RNA as its genetic material, never both. The reason is evolutionary economy: a virus is a minimal parasite that hijacks host machinery, and carrying both types of nucleic acid would be redundant and energetically wasteful. Even retroviruses, which use an RNA genome and a DNA intermediate, have only RNA in the virion. So statement A is true.

2. Statement B: “Viroids were named by Pasteur.”

   This is historically wrong. Louis Pasteur worked on germ theory and rabies vaccines in the 19th century, but viroids — tiny, circular RNA molecules that infect plants — were discovered and named by Theodor O. Diener in 1971. Pasteur never encountered them. Statement B is false.

3. Statement C: “Scrapie disease of sheep is caused by proteins.”

   Scrapie is a classic example of a prion disease. Prions are misfolded proteins that induce other normal proteins to misfold, causing neurodegeneration. No nucleic acid is involved. So scrapie is indeed caused by proteins (prions). Statement C is true.

4. Statement D: “In six kingdom classification three domains are present.” …

Typhoid fever is a serious bacterial infection that spreads through contaminated food and water. Understanding its causative agent and diagnostic methods is crucial for both medical knowledge and exam preparation. Let's break down each statement.

1. Evaluating Statement I: Haemophilus influenzae causes the typhoid.

   • Typhoid fever is a systemic infection primarily caused by the bacterium Salmonella enterica serovar Typhi, commonly known as Salmonella typhi. This bacterium is transmitted through the ingestion of food or water contaminated with the feces of an infected person.
   • Haemophilus influenzae, on the other hand, is a different type of bacterium. It is known to cause a range of infections, including meningitis, pneumonia, epiglottitis, and other invasive diseases, particularly in children. It does not cause typhoid fever.
   • Therefore, Statement I is false.

2. Evaluating Statement II: Typhoid can be confirmed by widal test.

   • The Widal test is a serological test used to diagnose typhoid fever. It detects the presence of antibodies (agglutinins) in the patient's serum against the O (somatic) and H (flagellar) antigens of Salmonella typhi. …