Which of the following statements is FALSE regarding B cells?

Correct Answer: B

Rationale: B cells develop in the bone marrow, not the thymus. Firstly, hematopoietic stem cells in the bone marrow give rise to B cells. They undergo maturation and selection processes in the bone marrow, not in the thymus. Therefore, choice B is false. On the other hand, choices A, C, and D are correct. A is true because B cells with high affinity to self-antigens undergo apoptosis to prevent autoimmunity. C is true since B cells synthesize antibodies which can be displayed on their surface as B cell receptors. D is true because B cells can undergo class switching to produce different types of antibodies based on the immune response requirements.

Correct Answer: C

Rationale: The correct answer is C: adhesion. Adhesion refers to the attachment of microbes to surfaces in or on the body without causing harm. Infections (choice A) occur when microbes invade and multiply in the body, leading to harmful effects. Contamination (choice B) refers to the presence of unwanted or potentially harmful substances, not necessarily microbes. Disease (choice D) is a state of poor health caused by a specific agent, such as microbes, resulting in identifiable symptoms. In this context, adhesion best describes the presence of microbes without causing harm or leading to an infection.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This bacterium is the most common cause of bacterial pneumonia in adults. It is a leading cause of community-acquired pneumonia. Streptococcus pneumoniae is known for its ability to infect the respiratory tract and cause inflammation in the lungs leading to pneumonia. Mycobacterium tuberculosis causes tuberculosis, not pneumonia. Escherichia coli is associated with urinary tract infections and gastrointestinal illnesses, not pneumonia. Clostridium botulinum causes botulism, a severe form of food poisoning, but not pneumonia. Therefore, the correct choice is Streptococcus pneumoniae as it is specifically known for causing pneumonia.

Correct Answer: D

Rationale: The correct answer is D. Isoniazid inhibits pyridoxal phosphate synthesis, which is the active form of vitamin B6. Pyridoxal phosphate is essential for various enzymatic reactions, including neurotransmitter synthesis and myelin formation. Therefore, isoniazid-induced pyridoxal phosphate deficiency can lead to neurological symptoms like polyneuritis, paresthesia, memory disorders, and convulsions. A: Disruption of cell membrane synthesis - This is not the mechanism of isoniazid side-effects. B: Inhibition of protein synthesis - Isoniazid does not directly inhibit protein synthesis. C: Inhibition of RNA-synthesis - Isoniazid does not inhibit RNA synthesis; it affects vitamin B6 metabolism.

Correct Answer: A

Rationale: Ribosomes are responsible for protein synthesis, where they translate the genetic information from mRNA into specific amino acid sequences to form proteins. This process occurs in both prokaryotic and eukaryotic cells. Ribosomes do not participate in the breakdown of substances, which is mainly carried out by lysosomes or proteasomes. Oxidative phosphorylation involves the generation of ATP in mitochondria, not ribosomes. Ribosomes do not inactivate antibiotics; this is typically done by enzymes or chemical modifications. Therefore, the correct answer is A as ribosomes play a crucial role in protein synthesis.

Correct Answer: C

Rationale: The correct answer is C: IgA. IgA is the most prominent immunoglobulin class present in external secretions like saliva, tears, and mucus. It plays a crucial role in mucosal immunity by preventing pathogens from entering the body. IgG is mainly found in the blood and tissues, IgM is the first antibody produced in response to an infection, and IgE is involved in allergic reactions. IgA's presence in external secretions helps protect the body from infections at mucosal surfaces, making it the correct choice in this context.

Correct Answer: A

Rationale: The correct answer is A: Enteropathogenic E. Coli (EPEC). EPEC bacteria adhere to the intestine wall using a protein called intimin, leading to microvilli damage. This attachment disrupts the normal absorption function of the intestine. Summary of why other choices are incorrect: B: Enterohemorrhagic E. Coli (EHEC) primarily causes bloody diarrhea and does not specifically target microvilli. C: Enterotoxigenic E. Coli (ETEC) produces toxins that lead to watery diarrhea but does not directly target microvilli. D: Enteroinvasive E. Coli (EIEC) invades and replicates within intestinal cells, causing inflammatory diarrhea, but does not specifically damage microvilli.

Correct Answer: B

Rationale: The correct answer is B: The generation time of bacteria refers to the time for a cell to divide and double in number. This is because the generation time is the period required for one bacterial cell to undergo binary fission and divide into two identical daughter cells. This process involves DNA replication, followed by cell division, resulting in an increase in the number of bacterial cells. Incorrect choices: A: The time for DNA replication - While DNA replication is a crucial part of bacterial growth and reproduction, it does not solely define the generation time. C: The time for spore formation - Spore formation is a specific process in certain bacteria and is not synonymous with the generation time. D: The time for toxin production - Toxin production is an independent process in bacteria and does not directly relate to the generation time.

Correct Answer: B

Rationale: The correct answer is B: Herpangina. Family Parvoviridae includes Parvovirus B19, which causes Fifth disease, aplastic crisis in chronic anemia, and fetal infections. Herpangina is caused by Enteroviruses, not Parvoviridae. Parvovirus B19 specifically targets erythroid progenitor cells, leading to anemia in susceptible individuals. Fetal infections can result in hydrops fetalis. Aplastic crisis occurs in patients with chronic anemia due to suppression of erythropoiesis by the virus.