Correct Answer: A

Rationale: The correct answer is A: Trichomonas vaginalis. Trichomonas vaginalis is a protozoan parasite that causes trichomoniasis, a sexually transmitted infection characterized by inflammation of urogenital tracts. The description of big unicellular pyriform organisms with a sharp spike on the back end of their bodies, big nucleus, and undulating membrane matches the morphology of Trichomonas vaginalis. Trichomonas hominis and Trichomonas buccalis are different species that do not cause urogenital infections. Trypanosoma gambiense is a protozoan parasite causing African trypanosomiasis and does not match the described morphology or symptoms.

Correct Answer: B

Rationale: The correct answer is B: Staphylococcus aureus. This bacterium is commonly found in hospital environments due to its ability to colonize human skin and mucous membranes. It is a leading cause of hospital-acquired infections. Borrelia burgdorferi is mainly associated with Lyme disease transmitted by ticks, not commonly found in hospitals. Bacillus cereus is a foodborne pathogen, not typically prevalent in hospital settings. Staphylococcus haemolyticus is a coagulase-negative staphylococcus species, less pathogenic and less common in hospitals compared to Staphylococcus aureus.

Correct Answer: B

Rationale: The correct answer is B: Alpha-hemolytic streptococci. Streptococcus pneumoniae are classified as alpha-hemolytic streptococci based on their ability to partially break down red blood cells, causing a greenish discoloration on blood agar plates. This distinguishes them from beta-hemolytic streptococci, which completely break down red blood cells. Streptococcus pneumoniae are not resistant to bile (choice A), as they are bile soluble. They are not stained by the Ziehl-Neelsen staining method (choice C), which is used for acid-fast bacteria such as Mycobacterium tuberculosis. They are sensitive to optochin (choice D), which is a key differentiating factor in identifying them from other alpha-hemolytic streptococci.

Correct Answer: A

Rationale: Step 1: Gram-positive cocci in chains suggest Streptococcus species. Step 2: Catalase-negative eliminates Staphylococcus aureus. Step 3: Beta-hemolytic on blood agar matches Streptococcus pyogenes. Step 4: Streptococcus pyogenes is a common cause of sore throat (pharyngitis). Summary: A: Streptococcus pyogenes - Correct due to Gram-positive cocci in chains, catalase-negative, beta-hemolytic, and common cause of sore throat. B: Staphylococcus aureus - Incorrect due to catalase-positive. C: Streptococcus pneumoniae - Incorrect due to alpha-hemolytic. D: Enterococcus faecalis - Incorrect due to not being beta-hemolytic.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pyogenes. 1. Gram-positive cocci in chains suggest streptococci. 2. Beta-hemolytic activity indicates Streptococcus pyogenes. 3. Streptococcus pyogenes is commonly associated with sore throat (pharyngitis) and fever. 4. Streptococcus pneumoniae (choice B) is alpha-hemolytic and mainly causes pneumonia. 5. Enterococcus faecalis (choice C) is not typically associated with sore throat. 6. Staphylococcus aureus (choice D) is not commonly beta-hemolytic and is not a typical cause of pharyngitis.