CDS 2026 Previous Year GK Questions with Answers

The correct answer is A, Cell wall. Bacteria have a cell wall and human cells do not, so a drug that attacks the cell wall harms only the bacteria.

The bacterial cell wall is made of peptidoglycan, a mesh of sugars and short peptides that gives the cell its shape and stops it bursting. Penicillin blocks the enzymes that build the cross-links of this mesh. A growing bacterium then makes a weak wall, takes in water and bursts. Human cells have only a plasma membrane and no peptidoglycan, so they are left alone. Alexander Fleming discovered penicillin in 1928 from the mould Penicillium notatum.

B is wrong: human cells also have a plasma membrane, so attacking it would harm our cells too.

C is wrong: some antibiotics, such as tetracycline, act on bacterial ribosomes, but penicillin does not.

D is wrong: penicillin does not act on the bacterial chromosome or DNA.

Exam tip: penicillin, discovered by Fleming in 1928, works by blocking the bacterial cell wall.

The correct answer is D, Lactase. Lactase breaks down lactose, the sugar in milk, so a person who still makes enough of it can digest milk at any age.

Lactase is made by the cells lining the small intestine. It splits lactose into two simple sugars, glucose and galactose, which the body can then absorb. Babies produce plenty of it because milk is their only food, but in most people its production falls after weaning. When lactose is not digested, bacteria in the large intestine ferment it, causing gas, bloating and loose motions. This condition is called lactose intolerance.

A is wrong: cellulase breaks down cellulose, the fibre of plant cell walls, and humans do not make it at all.

B is wrong: amylase, found in saliva and pancreatic juice, digests starch, not milk sugar.

C is wrong: pectase acts on pectin in plant tissue and has no role in human digestion of milk.

Exam tip: lactase digests lactose, maltase digests maltose, sucrase digests sucrose, and amylase digests starch.