Which of the following is a key feature of Mycobacterium tuberculosis?

Correct Answer: C

Rationale: The correct answer is C: Acid-fast cell wall. Mycobacterium tuberculosis is characterized by its unique cell wall composition, containing high levels of mycolic acids which make it acid-fast. This property allows it to resist decolorization during acid-fast staining procedures. This distinguishes it from other bacteria. A: Producing spores is not a key feature of Mycobacterium tuberculosis. Spore formation is more commonly associated with bacterial species such as Bacillus and Clostridium. B: Mycobacterium tuberculosis is not Gram-negative. It is classified as acid-fast Gram-positive. D: Mycobacterium tuberculosis does not produce toxins as a key feature. Its pathogenicity is more closely related to its ability to evade host immune responses and persist in host tissues.

Correct Answer: C

Rationale: The correct answer is C: Macrolides or fluoroquinolones. Legionella pneumophila is a bacterium that causes Legionnaires' disease, a severe form of pneumonia. Macrolides (like azithromycin) and fluoroquinolones (like levofloxacin) are the recommended first-line treatments due to their ability to penetrate into lung tissues where Legionella infects. Penicillins and cephalosporins (choice A) are not effective against Legionella as it is intracellular and resistant to these antibiotics. Ethambutol and isoniazid (choice B) are used to treat tuberculosis, not Legionella infections. Second and third generation cephalosporins (choice D) are also not effective against Legionella. Therefore, the best choice for treating Legionella pneumophila infections is macrolides or fluoroquinolones.

Correct Answer: A

Rationale: The correct answer is A: HSV-1. Herpes simplex infections on lips and in the mouth are most commonly caused by HSV-1 due to its preference for infecting oral mucosa. HSV-2 typically causes genital herpes. HSV-6 and HSV-7 are less common and not typically associated with oral infections. Therefore, based on the typical presentation and epidemiology of herpes simplex infections, HSV-1 is the most likely cause in this scenario.

Correct Answer: B

Rationale: The correct answer is B: ELISA test for IgM anti-HAV in serum and HAV-Ag in stools. This test is the most reliable for detecting acute hepatitis A infection because IgM antibodies are produced early in the infection, indicating an active infection. Detection of HAV antigen in stools also confirms active viral shedding. A: Western blot assay for IgG anti-HAV in serum is not the most reliable for acute infection detection as IgG antibodies are produced later in the infection and indicate past exposure or immunity. C: ELISA test for IgG anti-HAV in serum is not the most reliable for acute infection detection as IgG antibodies are produced later in the infection and indicate past exposure or immunity. D: None of the above is incorrect as option B is the most appropriate choice for detecting acute hepatitis A infection.

Correct Answer: D

Rationale: The correct answer is D: BCG vaccination. BCG vaccination is recommended for individuals with a negative Mantoux test result, especially in countries where tuberculosis is prevalent. This vaccination can help provide protection against tuberculosis. Choice A is incorrect because repeating the Mantoux test in a month is unnecessary since the boy already had a negative reaction. Choice B is incorrect as serodiagnosis of tuberculosis is not the first-line approach for diagnosis, especially in this scenario. Choice C is incorrect because urgent isolation is not necessary based solely on the Mantoux test result. Isolation is typically reserved for individuals with active tuberculosis.

Correct Answer: A

Rationale: The correct answer is A: Giardia lamblia. This protozoan matches the description of the causative agent because it has two nuclei, four flagella, and a ventral adhesive disc. Giardia lamblia is commonly found in the duodenum and causes giardiasis. The other choices can be eliminated because Leishmania donovani causes visceral leishmaniasis, not the described symptoms. Entamoeba coli is a non-pathogenic amoeba found in the large intestine. Trypanosoma gambiense causes African trypanosomiasis and is transmitted by the tsetse fly, not matching the described characteristics.

Correct Answer: A

Rationale: The correct answer is A: Volutin granules. Volutin granules are metachromatic granules that stain dark blue with methylene blue or Leishman stain. In diphtheria-causing bacteria, Corynebacterium diphtheriae, these granules are typically seen as yellow rods with dark blue thickenings on their ends when stained. Volutin granules are storage inclusions composed of polymetaphosphate that serve as an energy reserve for the bacteria. The presence of these granules in the detected microorganisms indicates the characteristic feature of Corynebacterium diphtheriae. Summary of other choices: B: Plasmids are extrachromosomal DNA molecules that are not typically visible on stained smears. C: Capsules are outer protective layers that are not visible on stained smears. D: Spores are reproductive structures formed by certain bacteria, not typically seen in Corynebacterium diphtheriae.

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: C

Rationale: The correct answer is C: Acid-fast cell wall. Mycobacterium tuberculosis is characterized by its unique cell wall composition, containing high levels of mycolic acids which make it acid-fast. This property allows it to resist decolorization during acid-fast staining procedures. This distinguishes it from other bacteria. A: Producing spores is not a key feature of Mycobacterium tuberculosis. Spore formation is more commonly associated with bacterial species such as Bacillus and Clostridium. B: Mycobacterium tuberculosis is not Gram-negative. It is classified as acid-fast Gram-positive. D: Mycobacterium tuberculosis does not produce toxins as a key feature. Its pathogenicity is more closely related to its ability to evade host immune responses and persist in host tissues.