A fecal smear from a patient with severe diarrhea revealed motile Gram-negative rods with a comma shape. What is the likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Vibrio cholerae. This bacterium is the likely causative agent because it is known to cause severe diarrhea, and it is characterized by its motile, comma-shaped Gram-negative rods. Vibrio cholerae produces cholera toxin, leading to profuse watery diarrhea. Shigella dysenteriae causes dysentery with bloody diarrhea, Salmonella typhi causes typhoid fever, and Escherichia coli can cause various types of diarrhea but typically does not have a comma shape.

Correct Answer: B

Rationale: The correct answer is B: Agglutination reaction. This test is commonly used to detect specific antibodies or antigens. In this case, it can be used to detect plague antigens in the rat tissues or antibodies in the villagers. This test is rapid, sensitive, and can help establish the presence of the causative agent quickly. A: Ring precipitation reaction is not typically used for diagnosing plague and is more commonly used in other types of infections. C: Passive agglutination reaction is not specific for plague and is more commonly used for detecting specific antibodies in diseases like syphilis. D: Neutralization reaction is not typically used for diagnosing plague and is more commonly used to determine the presence of neutralizing antibodies in viral infections.

Correct Answer: A

Rationale: The correct answer is A: Β-lactamase. In ELISA, enzymes like alkaline phosphatase, horseradish peroxidase, and Β-galactosidase are commonly used as labels to detect the presence of specific molecules. Β-lactamase is not typically used in ELISA as it is an enzyme used to hydrolyze β-lactam antibiotics. Therefore, it does not serve the purpose of detecting specific molecules in ELISA. Alkaline phosphatase, horseradish peroxidase, and Β-galactosidase are preferred enzymes in ELISA due to their ability to produce a detectable signal when bound to specific molecules.

Correct Answer: A

Rationale: The correct answer is A: Contaminated. The air is never completely free of microorganisms, so it is considered contaminated. Microorganisms are ubiquitous in the environment. Permissible limits (B) imply a specific standard, which is not relevant to the general microbiological characteristic of air. Almost pure (C) and pure (D) are not accurate as air always contains some level of microorganisms.

Correct Answer: A

Rationale: The correct answer is A: Entamoeba histolytica. This is because it is the only choice that matches the description of cysts with four nuclei, characteristic of Entamoeba histolytica. Entamoeba histolytica is a protozoan parasite that causes amoebiasis, leading to symptoms like persistent diarrhea. The other choices can be eliminated for the following reasons: B: Balantidium coli - It is a ciliated protozoan that causes dysentery in humans but does not produce cysts with four nuclei. C: Giardia lamblia - It is a flagellated protozoan that causes giardiasis, but it forms cysts with only two nuclei. D: Trichomonas hominis - It is a flagellated protozoan found in the human intestine, but it does not produce cysts with four nuclei. Therefore, based on the description of cysts with four nuclei in the stool sample,

Correct Answer: D

Rationale: The correct answer is D: Blood. Normal flora refers to the microorganisms that naturally inhabit specific areas of the body. Blood is considered a sterile environment in healthy individuals, meaning it does not harbor any microorganisms under normal circumstances. In contrast, the intestine, skin, and vagina all have normal flora as they provide suitable environments for various microbial species to thrive. The presence of normal flora in these sites helps maintain a balance and protect against pathogenic invaders. Blood, being a vital and sterile fluid, is not meant to have any resident microbial populations.

Correct Answer: A

Rationale: The correct answer is A: Delayed-type hypersensitivity. The Mantoux test is a type IV hypersensitivity reaction, which is also known as delayed-type hypersensitivity. This reaction typically occurs 48-72 hours after exposure to an antigen, such as tuberculin in this case. The focal skin hardening and redness that appeared 60 hours after tuberculin introduction align with the timeline and characteristics of a delayed-type hypersensitivity reaction. Summary: - Choice A (Delayed-type hypersensitivity): Correct, as explained above. - Choice B (Immune complex-mediated hypersensitivity): Incorrect, as this reaction involves immune complexes and typically presents with tissue damage and inflammation. - Choice C (Complement-mediated cytotoxic hypersensitivity): Incorrect, as this reaction involves the activation of complement proteins leading to cell lysis. - Choice D (Immediate hypersensitivity): Incorrect, as this reaction, such as in allergies, occurs within minutes to hours of exposure to an antigen and involves IgE-mediated mast

Correct Answer: A

Rationale: The most likely causative agent is Corynebacterium diphtheriae. Step 1: Presence of rod-shaped bacteria indicates a bacillus. Step 2: Polar granules suggest metachromatic granules seen in C. diphtheriae. Step 3: C. diphtheriae causes diphtheria, which presents with sore throat. Summary: B is incorrect as S. pneumoniae is a cocci, not a bacillus. C is incorrect as H. influenzae is a pleomorphic coccobacillus. D is incorrect as K. pneumoniae is a large, encapsulated bacillus.

Correct Answer: D

Rationale: The correct answer is D: Cell cultures. Rotavirus diagnosis involves detecting viral antigens in stool samples using cell cultures. Cell cultures allow for the replication of the virus, facilitating its identification. A: Egg inoculation and B: Sensitive laboratory animals are not commonly used for Rotavirus diagnosis due to ethical concerns and practical limitations. C: Ascoli test is used for detecting specific proteins in serum, not for Rotavirus detection in stool samples. In summary, cell cultures are the most appropriate method for laboratory diagnosis of Rotavirus infection due to their ability to support viral replication and antigen detection.