Microscopic analysis of a smear stained by Ziehl-Neelsen method revealed bright red bacilli. What microorganism is being tested for?

Correct Answer: A

Rationale: The Ziehl-Neelsen stain is specific for acid-fast bacteria like Mycobacterium tuberculosis. The bright red bacilli seen in the smear indicate the presence of acid-fast organisms, which is characteristic of Mycobacterium tuberculosis. Staphylococcus aureus, Clostridium tetani, and Escherichia coli are not acid-fast bacteria and would not be stained red using the Ziehl-Neelsen method. Therefore, the correct answer is A: Mycobacterium tuberculosis.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This bacterium is known for producing a capsule that helps it evade the immune system. The capsule acts as a protective barrier, making it difficult for the immune system to recognize and attack the bacteria. Streptococcus pneumoniae is a common cause of pneumonia and meningitis. Explanation for why other choices are incorrect: B: Escherichia coli does not typically produce a capsule for immune evasion. It is more commonly associated with gastrointestinal infections. C: Salmonella enterica also does not produce a capsule for immune evasion. It is known for causing foodborne illness. D: Bacillus subtilis is a soil bacterium that is not known for producing a capsule to avoid the immune system.

Correct Answer: D

Rationale: The correct answer is D: Adenovirus. Adenovirus commonly causes acute febrile pharyngitis and pharyngoconjunctival fever. Adenovirus is known to cause respiratory infections and can present with symptoms such as sore throat, fever, and conjunctivitis. VZV (Varicella-Zoster Virus) causes chickenpox and shingles, not these specific symptoms. Measles virus causes a systemic illness with characteristic rash, not typically pharyngitis. Rubella virus causes a mild febrile illness with rash, but not pharyngitis. Therefore, based on the clinical presentation described, adenovirus is the most likely cause.

Correct Answer: A

Rationale: The correct answer is A: Borrelia burgdorferi. This bacterium is associated with Lyme disease, a tick-borne illness. Borrelia burgdorferi is transmitted to humans through the bite of infected black-legged ticks. It causes a range of symptoms, including fever, rash, joint pain, and neurological problems. Summary: B: Clostridium botulinum is associated with botulism, a serious illness caused by a toxin produced by this bacterium. C: Escherichia coli is a common bacterium found in the intestines of humans and animals, some strains can cause food poisoning. D: Streptococcus pneumoniae is a bacterium that can cause pneumonia and other respiratory infections, but not Lyme disease.

Correct Answer: C

Rationale: The correct answer is C: Thick peptidoglycan layer. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain during Gram staining. This thick layer is a key characteristic feature that helps differentiate Gram-positive bacteria from Gram-negative bacteria. Rationale: 1. Gram-positive bacteria retain the crystal violet stain due to their thick peptidoglycan layer. 2. This thick layer provides structural support and protection to the cell. 3. Gram-positive bacteria lack an outer membrane, which is a feature of Gram-negative bacteria (Choice A). 4. Thin peptidoglycan layer is a characteristic of Gram-negative bacteria, not Gram-positive (Choice B). 5. Presence of LPS (lipopolysaccharide) is a feature of Gram-negative bacteria, not Gram-positive (Choice D). In summary, the presence of a thick peptidoglycan layer is a characteristic feature of Gram-positive

Correct Answer: A

Rationale: The correct answer is A: Botulism. The tennis racket appearance seen under the microscope is characteristic of Clostridium botulinum, the bacterium responsible for botulism. This bacterium produces spores that resemble a tennis racket shape. Botulism is a type of food poisoning caused by the ingestion of the toxin produced by Clostridium botulinum. Summary: B: Salmonellosis is caused by Salmonella bacteria, which do not exhibit a tennis racket appearance. C: Cholera is caused by Vibrio cholerae, which also does not resemble a tennis racket under the microscope. D: Dysentery is typically caused by bacteria such as Shigella or Entamoeba histolytica, neither of which exhibit the characteristic tennis racket appearance.

Correct Answer: B

Rationale: The correct answer is B because retroviruses, including those in the Family Retroviridae, have a capsid consisting of two copies of single-stranded RNA (ssRNA). This is a defining characteristic of retroviruses, which use reverse transcriptase to convert their ssRNA into DNA. This DNA is then integrated into the host cell's genome. Choices A, C, and D are incorrect because Family Retroviridae viruses are not helical non-enveloped viruses (C), they are not helical enveloped viruses (A), and they do not have a complex type of symmetry (D).

Correct Answer: A

Rationale: The correct answer is A: Blood agar. Streptococcus pneumoniae is a fastidious bacterium that requires enriched media for growth. Blood agar provides the necessary nutrients and growth factors for the cultivation of S. pneumoniae, such as blood proteins and hemin. MacConkey agar (B) is selective for gram-negative bacteria and would not support the growth of S. pneumoniae. Peptone water (C) is a liquid medium that lacks the necessary nutrients for the growth of S. pneumoniae. Choice D is incorrect as blood agar is a suitable medium for cultivating S. pneumoniae.

Correct Answer: A

Rationale: Pili in bacteria are responsible for conjugation, allowing the transfer of genetic material between cells. They form a bridge between donor and recipient cells, facilitating the exchange of DNA. Flagella are for movement, capsules for protection, and ribosomes for protein synthesis. Pili is the only structure directly involved in genetic material exchange.