Giardiasis is a waterborne gastrointestinal disease caused by Giardia intestinalis, which is a:

Correct Answer: A

Rationale: Step 1: Giardiasis is caused by Giardia intestinalis, which is a well-known protozoan parasite. Step 2: Protozoans are single-celled eukaryotic organisms commonly found in water sources. Step 3: Giardia intestinalis infects the intestine, causing symptoms like diarrhea and abdominal cramps. Step 4: Bacteria (Choice B) are prokaryotic organisms, different from protozoans. Step 5: Fungi (Choice C) are also eukaryotic but are not responsible for causing Giardiasis. Step 6: Viruses (Choice D) are not cellular and cannot independently cause Giardiasis. In summary, the correct answer is A (protozoan) because Giardia intestinalis is a protozoan parasite specifically known to cause Giardiasis, while the other choices (Bacterium, Fungus, Virus) do not match the characteristics of the

Correct Answer: B

Rationale: The correct answer is B: Outer membrane with lipopolysaccharides. Gram-negative bacteria have an outer membrane that contains lipopolysaccharides, which play a role in pathogenicity and resistance to certain antibiotics. This outer membrane is a distinguishing feature of Gram-negative bacteria, as opposed to Gram-positive bacteria that have a thick peptidoglycan layer. Choices A, C, and D are incorrect because Gram-negative bacteria have a thin peptidoglycan layer, do have a cell wall (although it is different from Gram-positive bacteria), and do not contain teichoic acids.

Correct Answer: D

Rationale: The correct answer is D because antigens can be composed of lipids, proteins, or polysaccharides. Antigens are molecules that can elicit an immune response in the body. They can be diverse in their chemical composition, leading to the recognition and response by the immune system. Therefore, the correct choice is D as antigens can be any of these chemical components. Choices A, B, and C are incorrect because they each represent a possible chemical composition of antigens, but the flexibility of antigens to be made up of any of these components makes D the most accurate choice.

Correct Answer: A

Rationale: The presence of acid-fast bacilli stained red against a blue background indicates mycobacteria, specifically Mycobacterium tuberculosis in this case. Ziehl-Neelsen staining is commonly used to detect tuberculosis. Diphtheria is caused by Corynebacterium diphtheriae, which does not stain with Ziehl-Neelsen. Pneumonia is a general term for lung infections caused by various pathogens. Leprosy is caused by Mycobacterium leprae, which requires special staining methods different from Ziehl-Neelsen.

Correct Answer: A

Rationale: The correct answer is A: Ziehl-Neelsen staining method was applied. 1. Ziehl's solution is used as a primary stain in Ziehl-Neelsen staining for acid-fast organisms like Mycobacterium tuberculosis. 2. Sulfuric acid is used as a decolorizer to remove excess stain. 3. Methylene blue is not part of the Ziehl-Neelsen staining method. 4. Burri's, Gram's, and Peshkov's staining methods are not used for acid-fast staining.

Correct Answer: C

Rationale: The correct answer is C: Tularemia. Ticks can transmit tularemia, a bacterial infection caused by Francisella tularensis. Ticks pick up the bacteria when feeding on infected animals and can then transmit it to humans through their bites. Botulism (A) is caused by toxins produced by Clostridium botulinum bacteria, not transmitted by ticks. Anthrax (B) is a bacterial infection caused by Bacillus anthracis, typically transmitted through contact with contaminated animal products. MRSA (D) is a bacterial infection caused by Staphylococcus aureus, commonly transmitted through direct contact with infected individuals or contaminated surfaces, not by ticks.

Correct Answer: B

Rationale: The correct answer is B: Carrying antibiotic resistance genes. Bacterial plasmids are small, circular DNA molecules that can carry genes, including those for antibiotic resistance. This allows bacteria to survive exposure to antibiotics. Plasmids do not directly regulate bacterial growth (A), produce toxins (C), or facilitate endospore formation (D). Plasmids play a crucial role in the spread of antibiotic resistance among bacteria.

Correct Answer: A

Rationale: The correct answer is A: Picornaviridae. Picornaviruses, such as enteroviruses, are commonly found in the feces of infected individuals. Therefore, their presence in water indicates fecal contamination and the need for additional purification. B: Herpesviridae are not typically associated with fecal contamination. C: Flaviviridae are mainly transmitted through arthropods like mosquitoes, not fecal contamination. D: Retroviridae, including HIV, are not typically excreted in high levels in feces and are not indicators of fecal contamination.

Correct Answer: A

Rationale: The correct answer is A: Neisseria meningitidis. This is because Neisseria meningitidis is a Gram-negative diplococcus that is oxidase-positive and fermentative. Neisseria gonorrhoeae is also a Gram-negative diplococcus, but it is not fermentative. Moraxella catarrhalis is oxidase-positive but not fermentative. Haemophilus influenzae is Gram-negative but is not a diplococcus, and it is not fermentative. Therefore, based on the characteristics provided in the question, Neisseria meningitidis is the most likely causative agent.