Explanation: Chemical-assay methods are generally more accurate and require less time than biological methods, but they are less sensitive, and caution must be used lest biologically inactive degradation products give misleading results.

Explanation: The international unit of penicillin is the amount of activity produced under defined conditions by 0.5988 µg of the International Standard, which is a sample of pure benzyl-penicillin.

Explanation: Small paper disks impregnated with known amounts of chemotherapeutic agents are placed upon the surface of an inoculated plate. So after incubation in disk-plate methods, a zone of inhibition around the disk indicates that the organism was inhibited by the drug.

Explanation: It has been suggested that pigs respond dramatically to the addition of oxytetracycline to their diet because the antibiotic inhibits the growth of Clostridium perfringens in their intestines and prevents a chronic but subclinical toxemia.

Explanation: Streptomycin may have a “sparing effect” on the B12 in the diet, making it available in greater quantities for utilization by the animals.

Explanation: Some antibiotics are effective against plant pathogens and are attractive for the treatment of plant diseases. The extent of this practice is limited mainly by economic factors, i.e, the cost of the antibiotic.

Explanation: Nystatin is an antifungal agent produced during fermentation by a strain of Streptomyces noursei.

Explanation: Griseofluvin is used in the treatment of many superficial fungal infections of the skin and body surfaces and is also effective in the treatment of systemic mycoses.

Explanation: Acycloguanosine is a nucleoside analog that is active against the herpes virus in animals. Its mode of action appears to be that of inhibition of nucleotide utilization.

Explanation: The antiviral action of interferons is attributed to interference of protein synthesis. Interferons is among the most promising chemotherapeutic agents for treating viral diseases.

Explanation: Amantadine is a low-molecular weight compound which is very effective against influenza A virus; it is not effective against influenza B. The incidence of influenza A infection is greatly reduced by the use of this drug.

Explanation: Isonicotinic acid Hydrazide or Isoniazid has proved to be very useful in the control of tuberculosis in humans and is more effective when given alternately with streptomycin.

Explanation: The anthramycin group (anthramycin, sibromycin, tomaymycin and neothramycin) is an example of potent antitumor agents. Their antitumor activity is directed towards DNA structure and function.