The following statement refers to 1,25-dihydroxyvitamin D3 (calcitriol):

Correct Answer: D

Rationale: The correct answer is D. A: 1,25-dihydroxyvitamin D3 (calcitriol) is the active form of vitamin D and is the metabolite of choice for rapid action in raising serum calcium levels. B: Calcitriol does raise serum phosphate levels, but this effect may not be immediately observed. C: Calcitriol undergoes enterohepatic circulation, which means it is reabsorbed in the intestines and recycled through the liver. Therefore, all statements A, B, and C are true about 1,25-dihydroxyvitamin D3 (calcitriol), making option D the correct choice.

Correct Answer: C

Rationale: The correct answer is C: Erythromycin. Erythromycin is a macrolide antibiotic due to its macrocyclic lactone ring structure. Macrolides inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit. Neomycin (A) is an aminoglycoside, Doxycycline (B) is a tetracycline, and Cefotaxime (D) is a cephalosporin, none of which are macrolides. Thus, Erythromycin is the only drug among the options that belongs to the antibiotics-macrolides category.

Correct Answer: A

Rationale: Step-by-step rationale: 1. Isoniazid is a first-line drug specifically used to treat mycobacterial infections like tuberculosis. 2. It works by inhibiting cell wall synthesis of mycobacteria. 3. Streptomycin, Rifampin, and Kanamycin are also used for mycobacterial infections, but they are effective against a broader range of bacteria. 4. Streptomycin targets protein synthesis, Rifampin inhibits RNA synthesis, and Kanamycin disrupts protein synthesis. 5. Therefore, choice A, Isoniazid, is the correct answer as it targets mycobacteria specifically.

Correct Answer: B

Rationale: The correct answer is B because piperazine works by blocking acetylcholine transmission at the myoneural junction, leading to paralysis of helminthes. This impairs the ability of the worms to move and causes them to be expelled from the body. The other choices are incorrect because: A: Inhibiting microtubule synthesis and impairing glucose uptake is not the mechanism of action of piperazine. C: Inhibiting oxidative phosphorylation is not how piperazine works. D: Increasing cell membrane permeability for calcium leading to paralysis and death is not the mechanism of action of piperazine.

Correct Answer: B

Rationale: Step-by-step rationale: 1. Vincristine is a vinca alkaloid, not a nitrosourea derivative. 2. Carmustine, Lomustine, and Semustine are all nitrosourea derivatives. 3. Therefore, the correct answer is B (Vincristine). Summary: Vincristine is not a nitrosourea derivative, unlike Carmustine, Lomustine, and Semustine.