A

As spoken by the Fourth Doctor, A (Morse code: .- (TV: The Man Who Never Was)) was the first letter of the alphabet, preceding B. (TV: The Masque of Mandragora)

As of 25 December 2006, about one-third of Earth's human population had A-positive blood. (TV: The Christmas Invasion)

A levels were a type of exam. (TV: Terror of the Autons, Dragonfire)

In November 1963, science teacher Ian Chesterton set his Coal Hill School class a problem with the letters A, B and C representing the three dimensions. However, Susan Foreman pointed out that he had not acknowledged two additional dimensions: D, which Ian reasoned was time, and E, which Susan revealed was space. (TV: An Unearthly Child) While under mental attack by the Mandragora Helix, the Fourth Doctor and Sarah Jane Smith spoke the alphabet backwards to concentrate. (TV: The Masque of Mandragora)

When trying to remember "Young Parsons'" name, Professor Chronotis went through the alphabet with the Doctor and Romana II. They started with A, B, and C and eventually got to P, Q, and R before he skipped to X and Y. (TV: Shada, AUDIO: Shada)

As recorded in the Dalek Survival Guide, the 2150s Dalek invasion of Earth involving the First Doctor was part of a version of history "A" relative to "B" and "C". (PROSE: Dalek Survival Guide)

Whilst the pair were surrounded by GeoComTex security guards within Henry van Statten's vault, Rose Tyler quipped that the Ninth Doctor would be "Exhibit A" in a collection of aliens. (TV: Dalek)

"A" was among the letters written on a set of wooden blocks in Room 802 of Albion Hospital, within which the Empty Child was questioned by Dr Constantine. (TV: The Doctor Dances)

In 2028, under the influence of Nobody No-One, a cardiogram once sung the alphabet from A to P. (AUDIO: A Death in the Family)

The Tenth Doctor and the entity possessing Sky Silvestry spoke the first fifteen letters of the alphabet simultaneously. (TV: Midnight)

In 2010, Amy Pond wore a gold necklace in the shape of the letter "A". (TV: The Eleventh Hour et al.)

Upon seeing the image of Joseph Serf, actually a hologram, Clyde Langer announced that he was "preparing to say the A word", clarifying to Sky Smith that he meant aliens. (TV: The Man Who Never Was)