MCQ of the Day: 9 November 2026

Answer: B. McFadyean (M'Fadyean) reaction. Cutaneous anthrax follows contact with infected animals or their hides. Polychrome methylene blue (ripened Loeffler's methylene blue) stains the poly-gamma-D-glutamic acid capsule of Bacillus anthracis pink, around blue-black, square-ended bacilli; this is the McFadyean reaction, the classic rapid smear test for capsulated anthrax bacilli in clinical or carcass material.

A. The Quellung (Neufeld) reaction is apparent capsular swelling when organisms such as Streptococcus pneumoniae are mixed with type-specific antiserum; it uses antibody, not polychrome methylene blue.

C. The Ascoli test is a thermoprecipitation reaction in which heat-extracted antigen from tissue (often putrefied hides) forms a ring with anti-anthrax serum. It is serological, not a stained smear.

D. The string-of-pearls reaction describes chains of swollen, spherical B. anthracis cells after a few hours' growth on agar containing low-dose penicillin. It is a culture-based test, not direct microscopy of clinical material.