In each part of this question, list clinical examination findings for each of the two underlined conditions that would help you to distinguish between them:

  • Aortic regurgitation or mitral stenosis as the cause of a patient’s diastolic murmur.

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College Answer

Aortic regurgitation

  • Collapsing pulse / wide pulse pressure
  • Decrescendo murmur heard over left 3rd intercostal space parasternally
  • Murmur loudest sitting forward in expiration
  • Signs associated with large pulse volume and peripheral vasodilation; eg Corrigans, De Musets. Quinckes, Duroziez.
  • Evidence of associated conditions; Infective endocarditis, ankylosing spondylitis, other seronegative arthropathies, Marfans.
  • Soft 2nd heart sound
  • 3rd heart sound
  • Displaced apex beat
  • Signs of LV failure

Mitral stenosis

  • Malar flush
  • Atrial fibrillation
  • Small pulse pressure
  • Loud 1st heart sound
  • Opening snap
  • Low-pitched, rumbling diastolic murmur over apex loudest in left lateral position
  • Pulmonary hypertension

Discussion

This question comes straight from Talley and O'Connor.

Specifically, the part of Chapter 3 (The Cardiovascular System) titled "Correlation of physical signs and cardiovascular disease". The sections dealing with aortic regurgitation and mitral stenosis make several statements to distinguish the two murmurs.

Unfortunately, its not as though there is a summary of it anywhere, nor is there any sort of table with the features of the various murmurs, which one might use as a quick reference. In fact the college answer is the closest thing to a summary of this issue.

Fortunately, before all these new-fangled gadgets came about, there were real physicians, who knew how to auscultate a praecordium. I have an 1958 article which contains precisely this sort of table, comparing the two murmurs according to their features. Together with the UpToDate page on heart sounds, this table has formed the basis of my "model answer". For more details, one may refer to the Circulation Research article by Sabbah et al (1976). In Aubrey Leatham's 1954 article for the Lancet, one may also find various causes of split heart sounds. Another important reference is  Walker's Clinical Methods, in which  Crowley's Chapter 27 is dedicated entirely to diastolic mumurs.

 
Features which Distinguish Aortic Regurgitation from Mitral Stenosis
Feature Aortic regurgitation Mitral stenosis
Rhythm usually sinus usually AF
First heart sound Normal S1 Loud S1
Second heart sound Soft S2 (A2) Normal S2 (M2)
Additional sounds Third heart sound (S3) Opening snap
Pulse quality Collapsing pulse Small pulse pressure
Where is it loudest 3rd intercostal space, parasternally Apex
When is it loudest Sitting forward, during expiration In a left lateral position
Pitch, quality Decrescendo low-pitched, rumbing
Associated findings

Signs of LV failure,

displaced apex beat

"Malar flush"
Coexisting disease Infective endocarditis, ankylosing spondylitis, other seronegative arthropathies, Marfans. Pulmonary hypertension

In case anybody is interested, here are the eponymous signs of aortic regurgitation which were mentioned by the college. If any of you young people are wondering why you (probably) have never heard of these, it is because they are almost completely useless.

Corrigans sign: a "jerky" carotid pulse: full expansion, followed by complete collapse. You're palpating the pressure of the left ventricle, essentially. It's named after a 19th century Irishman. It indicates a severe aortic incompetence. 

de Musset's sign which the college answer has spelled incorrectly is  a visible nodding of the head in time with arterial pulsation in patients with severe aortic insufficiency. It is named after an aortically insufficient French poet.

Quincke's sign, otherwise known as Quincke's pulse, is a nail sign: it is seen when the nailbed is blanched. The pale nail bed flashed red and white as capillary refill is restored. It can also be seen in the absence of any aortic problems, in patients who have sclerodactily.

Duroziez's sign is elicited by listening over the femoral artery with the bell of the stethoscope. It is supposed to be a double murmur. According to some recent evidence, it has almost 100% specificity. There is supposed to be both a systolic and a diastolic bruit, as blood rushes into - and then rapidly out of - the femoral artery.

References

References

Nicholas Joseph Talley, Simon O'Connor; Clinical Examination: A Systematic Guide to Physical Diagnosis (7th ed)

SEGAL, JACK P., W. PROCTOR HARVEY, and MICHAEL A. CORRADO. "The Austin Flint murmur: its differentiation from the murmur of rheumatic mitral stenosis." Circulation 18.5 (1958): 1025-1033.

Leatham, Aubrey. "Splitting of the first and second heart sounds." The Lancet 264.6839 (1954): 607-614.

Sabbah, HANI N., and PAUL D. Stein. "Investigation of the theory and mechanism of the origin of the second heart sound." Circulation research 39.6 (1976): 874-882.

Saberi, Asif, and Saeed A. Syed. "Corrigan’s sign." Hospital Physician (1999): 29.

DAVIES, M., and A. Hollman. "de Musset sign." Heart 82.3 (1999): 262.

Norton, S. A. "Keratoderma with pseudo-Quincke's pulse." Cutis 62.3 (1998): 135-136.

Sapira, J. D. "Quincke, de Musset, Duroziez, and Hill: some aortic regurgitations." Southern medical journal 74.4 (1981): 459-467.

Luisada, Aldo A. "On the pathogenesis of the signs of Traube and Duroziez in aortic insufficiency. A graphic study." American Heart Journal 26.6 (1943): 721-736.

BLUMGART, HERRMAN L., and A. CARLTON ERNSTENE. "Two mechanisms in the production of Duroziez's sign: their diagnostic significance and a clinical test for differentiating between them." Journal of the American Medical Association 100.3 (1933): 173-177.