Microorganisms and Disease Questions and Answers

Answer: D

The toxin without ADP-ribosyl transferase activity is S. aureus α-toxin.

α-toxin (α-haemolysin) of Staphylococcus aureus is a pore-forming toxin: its monomers assemble into a heptameric beta-barrel pore in the target cell membrane, letting ions leak and lysing the cell. It has no enzymatic activity.

The other three are ADP-ribosylating toxins. Diphtheria toxin and Pseudomonas exotoxin A both ADP-ribosylate elongation factor 2 and halt protein synthesis, and pertussis toxin ADP-ribosylates the inhibitory G protein G_i, uncoupling it from its receptors.

Answer: B

The correct match is P-2, Q-1, R-4, S-3.

• Tetracyclines bind the 30S subunit and block aminoacyl-tRNA from entering the A site (P-2).
• Chloramphenicol binds the 50S subunit and inhibits peptidyl transferase, so peptide bonds cannot form (Q-1).
• Cycloheximide acts on eukaryotic 80S ribosomes, blocking translocation at the peptidyl transferase centre, and does not affect bacteria (R-4).
• Streptomycin binds the 30S subunit, causes misreading of the code and interferes with initiation (S-3).

Option A swaps cycloheximide and streptomycin, and C and D misplace tetracycline, which acts at the A site.

Answer: C

The correct match is P-II, Q-IV, R-I, S- III.

• African sleeping sickness is caused by Trypanosoma brucei (P-II).
• Rocky Mountain spotted fever is caused by Rickettsia rickettsii, spread by ticks (Q-IV).
• Mumps is caused by a paramyxovirus of the genus Rubulavirus (R-I).
• Lymphatic filariasis is caused by Wuchereria bancrofti (S-III).

Leishmania donovani (V) causes visceral leishmaniasis (kala-azar), which is not in Group I. The other options assign trypanosomes to mumps or rubulavirus to sleeping sickness.

Answer: C

The correct match is (P)-(i), (Q)-(iv), (R)-(ii), (S)-(iii).

• Streptococcus pyogenes, group A streptococcus, causes scarlet fever through its erythrogenic (pyrogenic) toxins (i).
• Brucella species cause brucellosis, known as undulant fever for its rising and falling temperature (iv).
• Mycobacterium tuberculosis infecting the spine causes Pott's disease, tuberculous spondylitis (ii).
• Enterotoxigenic Escherichia coli is the commonest cause of traveler's diarrhea (iii).

Options A and B give scarlet fever to the wrong pathogen and Pott's disease to S. pyogenes, and option D swaps the diseases of M. tuberculosis and E. coli.

Answer: A

Atypical pneumonia is caused by Chlamydia pneumoniae.

'Atypical' pneumonia is the milder, walking pneumonia with a dry cough, patchy interstitial changes on X-ray and no response to beta-lactams. It is caused by organisms that lack a typical cell wall or live inside cells: Chlamydia (Chlamydophila) pneumoniae, Mycoplasma pneumoniae and Legionella.

Streptococcus pneumoniae, Klebsiella pneumoniae and Haemophilus influenzae cause typical bacterial (lobar or bronchial) pneumonia.

Answer: C

The pioneer of chemotherapy was Paul Ehrlich.

Ehrlich sought a 'magic bullet', a chemical that would kill a pathogen without harming the patient. His systematic screening of arsenic compounds produced Salvarsan (arsphenamine) in 1909, the first effective synthetic drug against syphilis, and he coined the term chemotherapy.

Elie Metchnikoff discovered phagocytosis, Robert Koch established the germ theory with his postulates and identified the tubercle bacillus, and Ronald Ross showed that mosquitoes transmit malaria.

Answer: B

The correct match is (P)-(i), (Q)-(ii), (R)-(iv), (S)-(iii).

• Streptomycin, the first aminoglycoside, was isolated by Waksman's group from Streptomyces griseus.
• Bacitracin is a peptide antibiotic from Bacillus licheniformis (originally B. subtilis).
• Amphotericin B, a polyene antifungal, comes from Streptomyces nodosus.
• Chloramphenicol was first isolated from Streptomyces venezuelae.

The option that is otherwise identical swaps amphotericin B and chloramphenicol, and the others misplace streptomycin.

Answer: A

DOTS stands for directly observed therapy short-course.

Under DOTS (often written directly observed treatment, short-course), a health worker or trained volunteer watches the patient take each dose of the anti-tuberculosis drugs. The 'short-course' is the standard six-month multidrug regimen. Direct observation is meant to ensure the full course is completed, which cures the patient and prevents the emergence of drug-resistant TB.

The other expansions change the meaning: the strategy observes the therapy, not 'tuberculosis'; it is not a district-run system; and 'directly operated' does not describe anything the programme does.

Answer: C

The protozoal disease is kala-azar.

Kala-azar, or visceral leishmaniasis, is caused by the protozoan Leishmania donovani and spread by the bite of sandflies (Phlebotomus). The parasite multiplies in macrophages of the spleen, liver and bone marrow, causing fever, weight loss, an enlarged spleen and darkening of the skin.

The other three are bacterial: cholera is caused by Vibrio cholerae, gonorrhoea by Neisseria gonorrhoeae, and syphilis by the spirochaete Treponema pallidum.

Answer: C

The viral protease inhibitor is ritonavir.

HIV protease cuts the Gag and Gag-Pol polyproteins into mature viral proteins; without this, the particles that bud from the cell stay immature and non-infectious. Ritonavir blocks that protease. Because it also inhibits the liver enzyme CYP3A4, it is widely given in low doses to boost the levels of other protease inhibitors.

Acyclovir is a guanosine analogue that inhibits the DNA polymerase of herpesviruses. Zidovudine (AZT) is a nucleoside reverse transcriptase inhibitor that terminates DNA chains, and nevirapine is a non-nucleoside reverse transcriptase inhibitor that binds the enzyme away from its active site.