A patient with pneumonia had a sputum culture revealing Gram-positive diplococci. The bacteria were catalase-negative and alpha-hemolytic. What is the most likely causative agent?

Correct Answer: A

Rationale: Step 1: Gram-positive diplococci suggest a pneumococcal infection. Step 2: Catalase-negative indicates Streptococcus. Step 3: Alpha-hemolysis matches S. pneumoniae's characteristics. Therefore, Streptococcus pneumoniae is the most likely causative agent. Staphylococcus aureus (B) is catalase-positive, Neisseria meningitidis (C) is Gram-negative, and Klebsiella pneumoniae (D) is not diplococci or alpha-hemolytic.

Correct Answer: C

Rationale: The correct answer is C. Negri bodies are not associated with measles but with rabies. Step 1: Measles is a childhood infection with a rash (A). Step 2: Koplik spots are small white spots on a red base found inside the mouth and are characteristic of measles (B). Step 3: Negri bodies are eosinophilic cytoplasmic inclusions found in neurons infected with rabies virus, not measles (C). Step 4: Prophylaxis for measles is through the MMR vaccine, which is a live attenuated vaccine (D). Therefore, choice C is incorrect as it does not pertain to measles.

Correct Answer: A

Rationale: Step 1: Oval, round, elongated chains of gemmating cells indicate yeast-like budding cells seen in Candida species. Step 2: Candidosis is caused by Candida species, which is commonly found in the oral cavity and can cause infections on the tongue. Step 3: Candidosis presents as white patches or plaques on the tongue. Step 4: Therefore, the correct answer is A: Candidosis. Summary: B: Actinomycosis is caused by Actinomyces species, which are filamentous bacteria, not oval budding cells. C: Diphtheria is caused by Corynebacterium diphtheriae, which appears as gram-positive rods in chains, not oval budding cells. D: Staphylococcal infections are caused by Staphylococcus species, which appear as clusters of cocci, not oval budding cells.

Correct Answer: B

Rationale: Rationale: The correct answer is B. Viruses of the family Paramyxoviridae cause the formation of giant multinucleated cells called syncytia. This occurs when infected cells fuse together, resulting in large, multinucleated structures. This is a characteristic cytopathic effect of paramyxoviruses. Summary: A: Fine grained degeneration is not typically seen in paramyxovirus infections. C: Large cell degeneration with grape clustering of cells is not a typical cytopathic effect of paramyxoviruses. D: Paramyxoviruses do provoke a cytopathic effect, so this statement is incorrect.

Correct Answer: A

Rationale: The correct answer is A: Type IV hypersensitivity reaction. This type of reaction is characterized by a delayed response involving T cells. In this case, the appearance of a papule after 48 hours indicates a delayed hypersensitivity reaction, which is typical of a Type IV reaction. The other choices are incorrect because: B: Arthus phenomenon is a localized immune complex-mediated reaction, not seen in tuberculin tests. C: Seroreaction typically involves antibodies reacting with soluble antigens, not seen in this scenario. D: Type II hypersensitivity reactions involve antibodies targeting specific cells or tissues, which is not the mechanism at play in this case.

Correct Answer: D

Rationale: The correct answer is D: Immunocomplex. In this case, the patient's symptoms of fever, joint pains, rash, and itch along with leukopenia and thrombocytopenia indicate a serum sickness reaction. This is a type III hypersensitivity reaction where antigen-antibody complexes deposit in tissues, leading to inflammation and tissue damage. The symptoms typically occur 1-2 weeks after exposure to the antigen. Anaphylactic reaction (choice A) is immediate and involves IgE antibodies. Cytotoxic reaction (choice B) involves destruction of cells by antibodies. Delayed hypersensitivity (choice C) usually takes 48-72 hours to develop and involves T-cells. Therefore, the correct answer is D as it aligns with the symptoms and timeline of a type III hypersensitivity reaction.

Correct Answer: A

Rationale: The presence of flagella in a bacterial cell represents the H-antigen. Flagella are protein structures on the cell surface that play a role in bacterial motility. The H-antigen refers to the flagellar antigen, which is involved in immune responses and bacterial identification. Option A is correct because flagella are specifically associated with the H-antigen. Summarily, choices B and C are incorrect because the O-antigen and K-antigen are associated with lipopolysaccharide and capsule antigens, respectively, not flagella. Option D is incorrect as well because the correct association is with the H-antigen.

Correct Answer: A

Rationale: The correct answer is A because growth factors are essential nutrients that bacteria cannot synthesize on their own and must obtain from the environment for their growth. Bacteria rely on these external factors for processes like cell division, metabolism, and survival. Choice B is incorrect because bacteria can only synthesize growth factors in limited amounts, not sufficient for their growth. Choice C is incorrect as bacterial cells do not store growth factors in granules. Choice D is incorrect because bacterial cells do not synthesize growth factors in large amounts.

Correct Answer: D

Rationale: The correct answer is D because all the options are essential for a PCR reaction. A: DNA segment is required as the template for amplification. B: Two oligonucleotide primers are needed to initiate DNA replication. C: Heat stable DNA polymerase (not just heat stable DNA) is crucial for withstanding the high temperatures during PCR. In summary, without any of these components, the PCR reaction cannot proceed effectively.