A blood culture from a patient with sepsis revealed Gram-positive cocci in pairs. The bacteria were catalase-negative and alpha-hemolytic. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This is because the bacterium is Gram-positive cocci in pairs, catalase-negative, and alpha-hemolytic, which are characteristic features of S. pneumoniae. The other choices can be ruled out based on specific characteristics: B) Staphylococcus aureus is catalase-positive, C) Enterococcus faecalis is gamma-hemolytic, and D) Neisseria gonorrhoeae is a Gram-negative diplococcus. Therefore, based on the given information, S. pneumoniae is the most likely causative agent of the sepsis.

Correct Answer: A

Rationale: Step-by-step rationale for choice A being correct: 1. Penicillin allergy involves IgE antibodies binding to penicillin. 2. This triggers mast cell degranulation, releasing histamine. 3. Histamine causes allergic symptoms like rash, swelling. 4. This immune response characterizes Type I hypersensitivity reactions. Summary of other choices: B: Type II reactions involve antibody-mediated cell destruction, not seen in penicillin allergy. C: Type III reactions involve immune complex deposition, not typical in penicillin allergy. D: Type IV reactions are delayed hypersensitivity reactions, not immediate like penicillin allergy.

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: Trichomonas vaginalis. Trichomonas vaginalis is a sexually transmitted protozoan parasite that causes trichomoniasis, characterized by foamy purulent vaginal discharges. Microscopy of vaginal discharge showing flagellated protozoa with an undulating membrane and pear-shaped structure is indicative of Trichomonas vaginalis. Trypanosoma gambiense causes African sleeping sickness and is transmitted by tsetse flies. Leishmania donovani causes visceral leishmaniasis transmitted by sandflies. Lamblia intestinalis causes giardiasis and is characterized by diarrhea and flatulence.

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

Correct Answer: C

Rationale: Rationale: 1. Bacillus anthracis is a spore-forming bacterium that can survive for decades in the soil. 2. The spores are resistant to harsh conditions and can remain dormant for long periods. 3. This resilience allows Bacillus anthracis to persist in the old cattle burial ground for over 50 years. 4. Mycobacterium bovis, Brucella abortus, and Francisella tularensis are not known for long-term survival in soil. Summary: - Mycobacterium bovis, Brucella abortus, and Francisella tularensis do not have the spore-forming ability. - Mycobacterium bovis causes tuberculosis in cattle. - Brucella abortus causes brucellosis in cattle. - Francisella tularensis causes tularemia in animals and humans.

Correct Answer: D

Rationale: The correct answer is D: Helicobacter pylori. Helicobacter pylori is a known bacterium associated with causing duodenal ulcers. The presence of Gram-negative curved bacteria in the biopsy along with a highly positive rapid urease test is characteristic of H. pylori infection. Vibrio cholerae causes cholera, not duodenal ulcers. Campylobacter fetus is associated with gastroenteritis, not duodenal ulcers. Acinetobacter baumannii is a nosocomial pathogen, not typically associated with duodenal ulcers. In summary, the unique combination of findings in this case points towards H. pylori as the most likely cause of the disease.

Correct Answer: B

Rationale: The correct answer is B: Helicobacter pylori. This bacterium is responsible for stomach acid formation by producing an enzyme called urease, which converts urea into ammonia and carbon dioxide, leading to an increase in stomach pH. Staphylococcus aureus (A) does not cause stomach acid formation. Bacillus cereus (C) and Clostridium botulinum (D) are not involved in stomach acid processes.

Correct Answer: C

Rationale: The correct answer is C: Mycobacterium tuberculosis. This bacterium is responsible for causing tuberculosis as it specifically infects the lungs and can spread to other parts of the body. Mycobacterium leprae (A) causes leprosy, Streptococcus pneumoniae (B) causes pneumonia, and Clostridium botulinum (D) causes botulism, making them incorrect choices for tuberculosis. Mycobacterium tuberculosis has unique characteristics and mechanisms that enable it to cause tuberculosis, such as its ability to evade the immune system and form granulomas in the lungs.