Toxicological Trivia, September 10, 2009.  A diagnosis of methemoglobinemia …: 
a) It is relatively easy when cyanosis is abrupt and lacking other semiological data;
b) It is more difficult if the presence of cyanosis is more insidious and overlaps with other underlying disease;
c) Both statements above are correct

Answer c. Read more in Spanish, Methemoglobinemia.