Streptococci are:

Correct Answer: D

Rationale: The correct answer is D because Streptococci are Gram-positive bacteria, typically spherical in shape, and range from 0.5-1 µm in size. A is incorrect as streptococci are not rod-shaped and are usually smaller in size. B is incorrect as they are not Gram-negative and are not typically of such small size. C is incorrect as streptococci are not Gram-negative and are not usually rod-shaped. Thus, based on their characteristics, the correct answer is D.

Correct Answer: A

Rationale: The correct answer is A: Clostridium botulinum and Staphylococcus aureus. Clostridium botulinum produces a potent neurotoxin causing botulism, commonly associated with improperly canned foods. Staphylococcus aureus produces toxins causing symptoms of food poisoning. Bacillus anthracis and Mycobacterium tuberculosis are not typically associated with food poisoning. Treponema pallidum and Neisseria gonorrhoeae are bacteria causing sexually transmitted infections. Escherichia coli and Klebsiella pneumoniae are not directly linked to food poisoning.

Correct Answer: A

Rationale: The correct answer is A: IgG. IgG is the most abundant class of immunoglobulins in the body, constituting about 75% of total immunoglobulins. IgG plays a crucial role in long-term immunity, neutralizing toxins, and enhancing phagocytosis. IgE is involved in allergic reactions, IgD is mainly found on B cells and functions as a receptor, and IgM is the first antibody produced in response to an infection but is less abundant compared to IgG. Therefore, IgG is the correct choice based on its abundance and diverse functions in the immune system.

Correct Answer: A

Rationale: The correct answer is A: Clostridium botulinum and Clostridium tetani. Both bacteria produce exotoxins that affect the nervous system. Clostridium botulinum produces botulinum toxin, causing botulism, leading to muscle paralysis. Clostridium tetani produces tetanospasmin, causing tetanus, leading to muscle stiffness and spasms. Choice B (Escherichia coli and Salmonella) are known for causing gastrointestinal infections, not specifically affecting the nervous system. Choice C (Bacillus anthracis and Listeria monocytogenes) produce toxins that primarily affect other body systems, not the nervous system. Choice D (Pseudomonas aeruginosa and Staphylococcus aureus) are associated with skin and respiratory infections, not neurological effects.

Correct Answer: D

Rationale: The correct answer is D because the catarrhal stage of whooping cough is characterized by mild respiratory symptoms such as mild coughing, sneezing, or a runny nose. During this stage, the cough may start to worsen but is not yet severe. A: Incorrect - Decrease in paroxysms of coughing is not seen in the catarrhal stage; it occurs later in the paroxysmal stage. B: Incorrect - Subsequent respiratory infection for many months after the onset of pertussis is not a characteristic of the catarrhal stage. C: Incorrect - Uncontrollable coughing followed by a high pitched 'whoop' sound is typical of the paroxysmal stage, not the catarrhal stage.