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Which is the correct usage please:
" I have ordered a / an MRI (magnetic resonance scan) and will review the patient following this."
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Actually, the rule is pretty simple: it depends on the *initial *** of the following word. If the sound is consonant, we use "a". If it is a vowel sound, we use "an".

an MP (pronounced "an em pee") = in Britain, a Member of Parliament

See? "A" Member (consonant sound) of Parliament.

What if the abbreviation had been like this:

an MOP (/an em ou pee/)
a MOP (/a mop/)

We do need to make the difference, don't we? ;-D

And this foolish joke leads me to:

NATO = North Atlantic Treaty Organization (pronounced as one word /neitou/)

a NATO member (/a neitou membr/)

It could also be "/an en ei tee ou membr/". The thing is either the abbreviations are pronounced letter by letter or as a single word, we must follow the a/an rule above.

Hope this helps! Emotion: smile

Comments  
Hi, you asked
Which is the correct usage please:
" I have ordered a / an MRI (magnetic resonance scan) and will review the patient following this."

Rule: With single letters and groups of letters that are pronounced as individual letters, be guided by the pronunciation: a B road, a TUC leader; but an A road, an FA Cup match, an SAS unit (assuming the abbreviations are not mentally expanded to their full forms, which would alter the "

Answer
I have ordered an MRI…

Reference
"a" Pocket Fowler's Modern English Usage

a, an, called the indefinite article (or, by some grammarians, determiner). In origin, a and its by-form an are versions of the Old English an meaning
‘one’. (1) Before all normal words or diphthongs an is required (an actor, an eagle, an illness, an Old Master, an uncle). Before a syllable beginning
in its written form with a vowel but pronounced with a consonantal sound, a is used (a eulogy, a unit, a use; a one, a once-only). Before all consonants
except silent h, a is usual: a book, a history, a home, a household name, a memorial service, a puddle, a young man; but, with silent h, an hour, an honour.

With single letters and groups of letters that are pronounced as individual letters, be guided by the pronunciation: a B
road, a TUC leader; but an A road, an FA Cup match, an SAS unit (assuming the abbreviations are not mentally expanded to their full forms, which would
alter the
Students: Are you brave enough to let our tutors analyse your pronunciation?
 Raul's reply was promoted to an answer.
always goes by the sound...not the letter.......an MRI