Which of the following organisms produces aflatoxin, a carcinogenic substance?

Correct Answer: C

Rationale: Step-by-step rationale: 1. Aspergillus flavus is a fungus known to produce aflatoxin. 2. Aflatoxin is a potent carcinogen found in contaminated food. 3. Candida albicans is a yeast species not associated with aflatoxin production. 4. Claviceps purpurea is a fungus that produces ergot alkaloids, not aflatoxin. 5. Staphylococcus aureus is a bacterium known for causing food poisoning, not aflatoxin production. Summary: Aspergillus flavus is the correct answer as it is the organism known to produce aflatoxin, a carcinogenic substance. Other choices are incorrect as they are not associated with aflatoxin production.

Correct Answer: C

Rationale: The correct answer is C because plasmids do not encode essential bacterial properties. They are non-essential, extra-chromosomal DNA molecules that can replicate independently within bacterial cells. Plasmids mainly contain genes for antibiotic resistance, genetic transfer, and other specific functions. Choice A is correct as many plasmids do carry antibiotic resistance genes. Choice B is also accurate as plasmids are indeed smaller, circular, and exist outside the bacterial chromosome. Choice D is true as plasmids can facilitate bacterial conjugation, allowing the transfer of genetic material between bacteria.

Correct Answer: A

Rationale: The correct answer is A: Clostridium botulinum. This bacterium is known for producing botulinum toxin, which causes botulism. It is an anaerobic, spore-forming bacterium commonly found in soil. Clostridium tetani causes tetanus, not botulism. Staphylococcus aureus can cause food poisoning but not botulism. Neisseria gonorrhoeae is responsible for gonorrhea, not botulism. Therefore, A is the correct answer due to the specific association of Clostridium botulinum with botulism toxin production.

Correct Answer: B

Rationale: The function of bacterial pili is to aid in adhesion. Pili are hair-like structures on the bacterial surface that help the bacteria attach to surfaces or other cells. This adhesion is crucial for colonization and infection. Pili do not provide motility, as that is typically done by flagella. Bacteria form spores for survival under harsh conditions, and protein synthesis is carried out by ribosomes, not pili. Thus, the correct answer is B, aiding in adhesion.

Correct Answer: D

Rationale: The correct answer is D because viral multiplication in cell culture can be detected by observing a specific cytopathic effect (A) caused by viral infection in cells and by using an immunofluorescence detection method (B) to visualize viral antigens within the cells. Choice C, the characteristic type of viral colonies, is incorrect as viral multiplication is not typically detected by observing viral colonies in cell culture. Option D is the correct choice as both A and B methods are commonly used to detect viral multiplication in cell culture.

Correct Answer: A

Rationale: The correct answer is A: Isoniazid. Isoniazid is known to cause peripheral neuropathy as a side effect, especially with prolonged use. It can lead to symptoms such as numbness, tingling, and weakness in the toes and fingers due to its neurotoxic effects. Rifampicin (B) is less likely to cause neuropathy. Ciprofloxacin (C) is not typically associated with neuropathy. Sodium salt of benzylpenicillin (D) is an antibiotic but is not known to cause polyneuritis.

Correct Answer: C

Rationale: Step 1: Togaviridae is a family of viruses known to have non-segmented (+)ssRNA as their genetic material. Step 2: The positive sense RNA means that the viral RNA can be directly translated into proteins by host machinery. Step 3: This RNA genome acts as mRNA, allowing for immediate translation of viral proteins. Step 4: Therefore, choice C is correct as it accurately describes the genetic nature of Togaviridae. Summary: - Choice A is incorrect because Togaviridae viruses do not have helical symmetry. - Choice B is incorrect because Togaviridae viruses do not have non-segmented (-)dsRNA. - Choice D is incorrect because Togaviridae viruses are enveloped viruses.

Correct Answer: B

Rationale: The correct answer is B: EIEC. This strain of E. coli can cause bloody diarrhea in children similar to shigellosis due to its ability to invade and replicate within the intestinal epithelial cells, leading to inflammation and bloody stools. ETEC (A) causes watery diarrhea due to enterotoxins. EAEC (C) causes persistent diarrhea. EPEC (D) causes attaching and effacing lesions on the intestinal mucosa without invasion.

Correct Answer: A

Rationale: The correct answer is A: Syphilis. The presentation of painless inguinal lymph nodes, along with a small-sized ulcer with thickened edges and a "laquer" bottom of greyish color on the genital mucous membrane, is classic for primary syphilis. The painless nature of the lymph nodes and the specific characteristics of the ulcer are key features of syphilitic infection. Primary syphilis is caused by the bacterium Treponema pallidum and typically presents with a single, painless ulcer known as a chancre. It is important to consider syphilis in the differential diagnosis of genital ulcers. Summary of other choices: B: Tuberculosis - Unlikely, as tuberculosis typically presents with pulmonary symptoms and does not cause genital ulcers. C: Lepra - Unlikely, as leprosy does not typically present with painless inguinal lymph nodes and the specific ulcer characteristics described. D: Trophic ulcer - Unlikely