A patient with dysentery had a stool sample that was cultured on Endo agar, resulting in colorless colonies. The bacteria were Gram-negative rods. What is the most likely causative agent?

Correct Answer: A

Rationale: Rationale: 1. Endo agar inhibits the growth of Gram-positive bacteria. 2. Colorless colonies on Endo agar indicate lactose non-fermenting bacteria. 3. Shigella dysenteriae is a lactose non-fermenting, Gram-negative rod, causing dysentery. 4. Salmonella typhi, Escherichia coli, and Proteus vulgaris are lactose fermenters, so they would produce pink colonies on Endo agar. Therefore, the most likely causative agent is Shigella dysenteriae.

Correct Answer: D

Rationale: The correct answer is D: Shigella dysenteriae I. This bacterium is known to produce Shiga toxin, which is responsible for the most severe bacillary infections. The toxin can cause bloody diarrhea and potentially life-threatening complications like hemolytic uremic syndrome. Shigella boydii, flexneri, and sonnei, while also causing bacillary dysentery, do not produce the same level of severity as Shigella dysenteriae I due to the absence of Shiga toxin.

Correct Answer: D

Rationale: The correct answer is D: active transport system. Permeases are specific carriers that actively transport molecules across a cell membrane against their concentration gradient, requiring energy input. This process is known as active transport, which is essential for maintaining cellular homeostasis and allowing the cell to accumulate specific molecules. Facilitated diffusion (A) and passive transport (B) both involve the movement of molecules down their concentration gradient without the need for energy input. Simple diffusion (C) also does not involve specific carriers and relies on the natural movement of molecules from areas of high concentration to low concentration. Therefore, the correct choice is D as permeases specifically function in active transport systems.

Correct Answer: C

Rationale: The correct answer is C. Prostatitis is inflammation of the prostate gland, leading to symptoms like painful urination, fever/chills, and weak urine flow. Painful urination is due to irritation of the urinary tract by the inflamed prostate. Fever/chills indicate an infection, common in prostatitis. Weak urine flow is due to obstruction caused by the inflamed prostate. Choice A is incorrect as muscle aches, sore throat, and swollen lymph nodes are not typical symptoms of prostatitis. Choice B is incorrect as painful joints and nausea are not characteristic symptoms. Choice D is almost identical to the correct answer but includes an extra symptom of painful joints, which is not associated with prostatitis.

Correct Answer: B

Rationale: Step-by-step rationale: 1. Fimbriae are hair-like structures on bacteria, not rope-like. 2. Fimbriae are smaller in diameter compared to flagella. 3. Fimbriae are used for adherence or conjugation, not flagella. Therefore, choice B is correct as it incorrectly describes fimbriae, while choices A, C, and D correctly characterize fimbriae.

Correct Answer: A

Rationale: The correct answer is A: Clostridium tetani. The presence of Gram-positive rods with terminal spores resembling drumsticks indicates Clostridium tetani, the causative agent of tetanus. This bacterium forms spores at one end, giving it a drumstick appearance under the microscope. Clostridium perfringens (B) and Clostridium botulinum (D) do not have terminal spores. Bacillus anthracis (C) forms spores but typically appears as a square-ended or bamboo stick shape. Therefore, based on the microscopy findings, Clostridium tetani is the most likely microorganism responsible for the wound infection.

Correct Answer: B

Rationale: The correct answer is B: Streptococcus mutans. This bacterium is associated with the formation of dental caries due to its ability to metabolize sugars and produce acids that demineralize tooth enamel. Streptococcus pneumoniae (choice A) is not typically associated with dental caries. Bacillus anthracis (choice C) causes anthrax, not dental caries. Clostridium botulinum (choice D) causes botulism, not dental caries. Streptococcus mutans' unique characteristics make it the correct choice for the bacteria associated with dental caries.

Correct Answer: C

Rationale: Step-by-step rationale for why choice C (Adenoviridae) is correct: 1. Adenoviruses commonly cause respiratory illnesses such as the common cold and bronchitis. 2. They are highly contagious and easily spread through respiratory secretions. 3. Adenoviruses are a common cause of viral conjunctivitis as well. 4. Other choices are incorrect because herpesviruses (choice A) typically cause different types of infections, papovaviruses (choice B) are not common respiratory pathogens, and poxviruses (choice D) are not typically associated with respiratory illnesses.

Correct Answer: B

Rationale: Gas gangrene is caused by Clostridium perfringens due to its ability to produce gas in tissues leading to necrosis. Other choices are incorrect: A (C. difficile causes diarrhea), C (C. tetani causes tetanus), and D (incorrect as B is the causative agent).